After noticing your thread, I felt the need to answer the question with a lot of detail. This post will definitely be a very long one, but if anybody is interested in learning how each of the fully-evolved starters work in battle, then it is definitely worth the read. Here is how I rank them from best to worst, all in order:
1. Kanto
Venusaur – Amazing sweeper in Harsh Sunlight, as it has the ability to learn Weather Ball, which becomes an extremely powerful Fire-type coverage move, and amazing ability in Chlorophyll, which doubles its speed in Harsh Sunlight. Its STAB Sludge Bomb is great for taking care of other Grass-types and Fairy-types and it can use Solar Beam, a very powerful special Grass-type attack in the Sun without charging. Served as a great staller in generation VI after obtaining a Mega Evolution with the ability Thick Fat. Great in Singles and Doubles on Sun teams. I love Venusaur and use it on my main VCG Doubles team with Groudon.
Charizard – Became even better in the Sun because of its hidden ability Solar Power, which boosts its Special Attack in the Sun at the cost of a bit of chip damage each turn. Dominated generation VI single-handedly because of the two powerful Mega Evolutions it received, and in generation VIII, it became a great Dynamax sweeper with the ability to boost it's own speed and the speed of its allies with Max Airstream, causes chip damage with its Gigantamax signature move, and can employ powerful Grass-type coverage against Water-types that threaten it. As a result, Charizard is great in Singles and Doubles.
Blastoise – Great bulky Shell Smash sweeper, which lowers its defenses in favor of more attack power and speed. Served as a great wallbreaker in generation VI after obtaining a Mega Evolution, making it stronger and giving it the ability Mega Launcher, which boosted several "blast" moves it could learn. In generation VIII, the addition of Dynamax makes Blastoise bulkier after Shell Smash and thus more dangerous, as it can hit very hard, faster, and becomes difficult to take down. Its signature Gigantamax move also does chip damage for four turns. Also serves as a good Rapid Spinner. In Doubles, Blastoise is also a good for Follow Me support.
All and all, because the Kanto starters are the originals, they receive a lot of support from the developers, which, in turn, made them the best starters over time because they dominate every format in most generations where they have received support.
2. Galar
Cinderace – One of the best starters even because of its high Attack and Speed stats, making it a great sweeper, as well as its hidden ability, Libero, which changes its type based on whatever move it uses, ensuring that it always hits with STAB. When it Gigantamaxes, its signature G-Max move ignores other Pokémon's abilities, making it dangerous. As a result, Cinderace is broken and is a mainstay in Singles. Even in Doubles, Cinderace finds its niche with its signature move Court Change, which switches field effects that benefit the opponent such as, for example, Light Screen, Reflect, etc. to your side of the field instead, which is good, as these effects are often abused in Doubles.
Rillaboom – Hidden ability Grassy Surge is great for automatically setting up Grassy Terrain upon switch in, boosting its Grass-type moves and helping its allies. Also learns Grassy Glide, a Grass-type move that has priority in Grassy Terrain, which gets more powerful when Rillaboom uses it because of STAB and the effects of Grassy Terrain, making Rillaboom hit first and extremely hard. Serves as an excellent wallbreaker. Also learns a ton of coverage moves. Because of this, Rillaboom is great in Singles and Doubles.
Inteleon – Might not enjoy the same level of use as its other starter brethren, but is still a good Pokémon. With its ability to learn Snipe Shot, which has an increased critical hit ratio, hidden ability Sniper, which greatly increases the damage done by critical hits, and Focus Energy, which increases the probability of landing a critical hit, when equipped with a Razor Claw or Scope Lens, which increases critical hit ratio even further, Inteleon could land critical hits 100% of the time to deal massive damage, making it extremely dangerous. Considering that critical hits ignore a Pokémon's stat changes, Inteleon could be powerful enough to KO practically any Pokémon that doesn't outright resist it. In addition, with Inteleon's extremely high Special Attack and Speed stats, along with a vast movepool that gives it a lot of coverage, Inteleon is naturally a good special sweeper and is even stronger in the Rain. However, its really bad defensive stats make it sort of risky to use if it doesn't KO everything it attacks.
I ranked them as a very close second because they can do well in battle without needing a bunch of support from the developers. However, they aren't number one because the Kanto starters often find themselves in high positions in the meta because of their support compared to the Galar starters, such as World Championship level competition, and can do more consistent damage offensively.
3. Hoenn
Blaziken – One of the greatest Pokémon of all time. Blaziken is amazing in the fact that it has high attack, allowing it to hit hard, and despite having average speed stats, its ability Speed Boost boosts its Speed every turn, which makes it a dangerous physical sweeper. It can also be used in many different ways, such as a powerful physical attacker with Flare Blitz and High Jump Kick, or as an effective physical Baton Passer with Swords Dance and its Speed Boosts. It can also be used as a Reversal sweeper after using Swords Dance and surviving a potential KO with a Focus Sash, which does much more damage the less HP it has. It also got a Mega Evolution in generation VI, which made it even stronger. This makes Blaziken one of the most broken Pokémon and it's one of my all-time favorites. Torchic is also, in my opinion, the cutest Pokémon of all time and I always keep a Blaziken on my team.
Swampert – Great bulky physical attacker with a great defensive type combination in Water/Ground. Defensively, the only thing it really needs to worry about is Grass-types. Could also be used as a good entry hazard setter because of its bulk. Swampert can also learn several utility moves to get players an edge in battle. It got even stronger when it got a Mega Evolution in generation IV, where its physical attack got even stronger and gained the ability Swift Swim, which made it an absolute monster physical sweeper in the Rain.
Sceptile – A Pokémon that should be good, as it has good stats, but suffers from unsymmetrical offensive statistics. Has many good physically attacking moves for STAB and type coverage, and even learns its signature move Leaf Blade, but has only an average Attack stat. It does, however, have a good Special Attack stat and very good Speed, but it's not the best special sweeper because it doesn't really learn the best special attacking moves. It did, however, get much better in generation IV when it received a Mega Evolution, making it even stronger and faster, and thus a better special sweeper. Its ability that it obtained after Mega Evolving, Lightning Rod, also made it great for switching in to Electric-type attacks or preventing its partner from being hit by such attacks in Doubles, but still, better options exist for this role without having to waste a Mega Evolution. Sceptile isn't bad, but it can definitely be much better should it get a larger special movepool.
I ranked the Hoenn starters as 3rd because Blaziken really was a huge game changer back in its day and Swampert was a feared Rain sweeper after Mega Evolving. Sceptile wasn't that great, but did get a little better with Mega Evolution and served a niche role as a negator of Electric-type attacks.
4. Kalos
Greninja – One of the greatest Pokémon of all-time in my opinion. Absolute monster in the generation VI games because of its ability Protean, which allows it to change its type based on the attack it uses, which makes it always hit with STAB. It is very fast and hits hard, making it a great special sweeper, but can also be used as a good physical sweeper as well. It's very unpredictable. Also has another version of itself, Ash-Greninja, which is just as strong as a major legendary Pokémon, which makes Greninja arguably one of the greatest starter Pokémon of all time. I always pick Froakie for my generation VI games and always use Greninja in whatever game I can. Amazing Pokémon.
Chesnaught – Great bulky physical attacker that can protect itself with its signature move Spiky Shield, which can damage Pokémon that make contact with it, set entry hazards such as Spikes, recover with Synthesis, attack with STAB with Drain Punch to gain health while doing damage, and learn Wood Hammer, a very strong STAB Grass-type attack with 120 power. It also learns a lot of physical moves that give it great coverage, something that Grass-type starters tend to lack. Personally, I never got the chance to ever use Chesnaught, but I hope it returns to the newer games so I can finally get that chance.
Delphox – Great fast and specially bulky special sweeper with a good and unique typing of Fire/Psychic. Learns good coverage moves and could boost its Special Attack with Calm Mind and make it even bulkier. You can pull shenanigans with it using Trick. Other than that, it's just a pretty straightforward special sweeper and is good at its role. I bred the Fenniken I received from Shuana in Pokémon Y and transferred it to Ultra Sun to have one in that game and it didn't disappoint at all.
Fun fact: The most interesting aspect of the Kalos starters that I believe many players overlook is that, when fully-evolved, the Kalos starters are thus far the first and only starters to have a double core in their typings. By "core," I am referring to the "rock-paper-scissors" aspect of a set of three typings. For example, the most well-known core is Water, Fire, and Grass, which all resist and hit each other super-effectively in a way akin to rock-paper-scissors. The same exact thing can be done for Dark, Fighting, and Psychic as well, which the fully-evolved starters each have as a secondary type. This means that if you can get each Kalos starter on the same Singles team, then you potentially have a strong team on your hands with a lot of type coverage, both offensively and defensively.
They are fourth on my list because despite Greninja completely overshadowing the rest of them, the rest are actually good Pokémon in their own right. Greninja was a huge game changer, but while the other two are good at what they do, they just did their roles basically without being game changers.
5. Sinnoh
Infernape – Great physical sweeper that can do a lot of damage with its Iron Fist ability, high Attack stat, high Speed, and several punch moves that gives its attacks a boost in power and provide it with extra coverage. It also works even better with an Expert Belt, which boosts the power of its super-effective attacks. I raised one of these in Pokémon Ultra Sun by transferring it from the older games. It's defenses, however, are quite poor, meaning that it is unlikely to take a hit, meaning that it must sweep to be effective.
Empoleon – Great bulky special attacker with a good and unique typing in Water/Steel, giving it a lot of resistances and ability to hit really hard in the Rain. Also has the potential to be good for utility such as by using Defog and Stealth Rock. Piplup was always my choice when playing the generation IV games and Pokémon Brilliant Diamond because I love Empoleon.
Torterra – Good bulky physical attacker that is slow enough to be good on Trick Room teams. Hits hard with STAB moves such as Earthquake and Wood Hammer, and also has good recovery with the move Synthesis. Because of its bulk, it can also be used as a good Stealth Rock setter.
In conclusion, while the Sinnoh starters are not bad by any means, they are 5th on the list as a set because none of them stand out in any meaningful way. They just simply perform their roles well without actually being game changers.
6. Alola
Incineroar – Bulky physical attacker that is a rather broken Pokémon in Doubles because of its ability Intimidate, which decreases the Attack stat of all the opponent's Pokémon on their side of the field, and has great support moves, which makes it a great teammate with any Pokémon. Learns Parting Shot, which lowers its target's Attack and Special Attack right before switching out, which is a great pivot move. It's signature move, Darkest Lariat, inflicts damage while ignoring the opponent's stat changes as well. This makes Incineroar the most used Pokémon in competitive World Championship teams. However, it's not really that special in Singles, but can be used effectively on Trick Room teams. I have an Incineroar on my main VCG Doubles team and it never disappoints. I hope it returns for generation IX.
Primarina – Bulky special attacker that has the potential to get even bulkier and stronger with Calm Mind. Substitute makes it even harder to deal with one on one. It can also be used as a good special wallbreaker too. Dynamaxing in generation VIII makes it even bulkier and harder to deal with too. It's a good Pokémon for Singles, but not really for Doubles based on my experience.
Decidueye – Not as popular as Incineroar or even Primarina in competitive battles, Decidueye is nonetheless great for in-game playthroughs. Decidueye is too slow to be used as a sweeper, too fast to be used in Trick Room, and doesn't hit hard enough to do serious damage to be an effective wallbreaker without boosting. Yet, when it does boost, it can be a decent wallbreaker in generation VIII with the move Poltergeist. However, what prevents it from being worse than Samurott is that Decidueye can be used as a good Pokémon to prevent switching with its signature move Spirit Shackle. Also, during an in-game playthrough, Decidueye is a great Pokémon because of its ability to land critical hits with Leaf Blade, with stacks with the effects of holding a Razor Claw/Scope Lens, and max affection/friendship feature. I love Decidueye, picked Rowlet for my Ultra Sun playthrough, and hatched a shiny Rowlet in Pokémon Sword.
Because Incineroar is great in doubles, Primarina is good in Singles, and Decidueye is good for in-game playthroughs, the Alola starters are solid in their roles. However, they are rather low on the list because, with the exception of Incineroar, the other starters are not really game changers.
7. Unova
Serperior – Despite looking lackluster because of its mediocre stats except for its high speed stat, it's actually one of the best special sweepers in the game. It's hidden ability Contrary allows it to sharply boost its Special Attack stat instead of lowering after each use after using Leaf Storm, making Serperior extremely deadly. Has decent coverage with Dragon Pulse and Hidden Power Fire, which of course hit much harder after boosting with Leaf Storm. Is broken in both Doubles and Singles. Snivy is always my choice when playing any Unova game and Serperior is one of my all-time favorite Pokémon.
Emboar – Poorly bulky physical attacker that can do more damage with recoil moves because of its hidden ability Reckless. It naturally learns Flare Blitz, which is extremely powerful, and can make good use of Wild Charge for Electric-type coverage. However, Emboar does not really excel at much else and is outclassed by other starter Pokémon of the exact same type, such as Infernape and especially Blaziken. Emboar's weak defenses practically negate its high HP, making its bulk rather frail.
Samurott – The only thing that it really has going for it is that it can be used as a special attacker or a physical attacker effectively. The problem with Samurott is that if you look up the definition of "painfully average," you'd see its picture. It's too slow to be a sweeper, but too fast to be effective in Trick Room. It doesn't hit hard enough, physically or specially, to be a wallbreaker unless it boosts with Swords Dance, and its ability Shell Armor, which prevents it from being hit by critical hits, is simply just useless. It is outclassed by several other Water-types that can do what Samurott can do and more. It might just be the second-worst starter of all time.
The Unova starters are low on my list because, despite Serperior being a great Pokémon, Emboar is outclassed by two other starter Pokémon of the same type and Samurott doesn't stand out in any form or fashion, nor does it really perform an effective role that many other Pokémon can't.
8. Johto
Feraligatr – Great physical sweeper and heavy hitter with its Sheer Force hidden ability equipped with a Life Orb. Has the ability to boost its speed and attack stats with Dragon Dance to become a good sweeper and learns lot of physical coverage moves such as Crunch, and Ice Punch to take down Pokémon of various types with a lot of power. Is really good in Singles, but pretty useless in Doubles, as much better water options exist in Doubles. Still, I love Feraligatr as one of my all-time favorite starters.
Typholsion – Good special sweeper with access to a very powerful Fire-type move in Eruption, as well as a good ability in Flash Fire that allows it to switch into Fire-type attacks, completely negate them, and increase the power of its own Fire attacks. Other than that, despite being a better special attacker than physical, nothing much about it stands out and much better Fire-types exist that can do things much better than it can.
Meganium – Defensive Pokémon with the ability to stall by using moves such as Toxic, Protect, and Giga Drain, and recovering health with Synthesis. Other than that, Meganium just isn't good at all and can't really be used effectively in any other role. Much better options exist for stall as well due to Grass-type not being good defensively in general because of its weaknesses. I hate to say it, but Meganium just might be the worst starter of all time. One thing it does have going for it is that it's still rather cute despite being fully evolved.
Fun fact: The final evolutions of the Johto starters all have the same stat lines as the Kanto starters, with a few having some minor differences. Meganium and Venusaur have the same stats, but have their defensive and offensive stats swapped. Feraligatr and Blastoise have the same stats as well, but also have their defensive and offensive stats swapped. Typholsion and Charizard, on the other hand, literally have the exact same stats entirely.
With that being said, the Johto starters, as a set, are easily the worst starters, but Feraligatr is really good and is one of my favorites. It's a shame that it has to be placed within such mediocre company. If Typholsion simply could do more than Eruption, negate Fire-type attacks, and had better coverage moves, and if Meganium wasn't a defensive version of Venasaur without any of Venasaur's strong suits that make it good, then the Johto starters would have been higher on my list.
