My favorite Pokemon is Larvitar, it has a simple design, it's pokedex entry says it can eat a mountain, and it can become Tyrannitar, one of the strongest dark type Pokemon ever, even though it is weak to fighting types, but thats why I have Golurk.
I still think my favorite overall Pokemon are Tyranitar, Raikou, Chandelure & Braviary. I try to use all of them in a game if I get the chance, or migrate them over at some point.
As for the rest of the types, going by primary type:
Fire: Gotta Be Arcanine here. One of the first Pokemon I really loved.
Water: Gyarados or Kingdra.
Grass: Cacturne is amazing. Scarecrow design is eerie and very well done.
Ice: Mamoswine. It's a wooly mammoth. What's not to like?
Normal: Ursaring. Bear Pokemon need more love.
Dark: Second favorite type. Gotta go with Honchkrow or Houndoom. Boss. B)
Psychic: I really like Gallade, and its knight/warrior motif.
Ground: Rhyperior. I love pretty much all the Gen IV evolutions.
Steel: Steelix, cool name and really cool design.
Bug: Scizor. It's like the definitive Bug type.
Dragon: Garchomp. One of the most BA Pokemon there is.
Fairy: So far, I actually really like Swirlix the most.
Poison: Toxicroak. It's a menacing toxic frog.
Fighting: Machamp. One of my original favorites, had a really strong one in RBY.
Vulpix is my favorite fire-type. It's cute and extremely cuddly. My favorite water-type is Suicune. I love its beauty and elegance. For grass, my favorite is Cherrim. Its smile is just simply adorable. Chansey is my favorite normal Pokémon. It's able to endure a lot of hits in battles, and when it's holding Eviolite, it could become even more vicious. Salamance is my favorite dragon. Either that, or Garchomp. My favorite ground-type Pokémon is Hippowdon. It's got heavy bulk, and has the ability to summon sandstorm as soon as it's out of its Poké Ball. Scizor is my favorite bug-type. It's quite powerful physically, and it has the ability to take out almost any Pokémon with just one Bullet Punch. Metagross is my favorite steel-type. I'm a big fan of its design. When it comes to rock, my favorite Pokémon is Aerodactyl. My favorite flying-type is probably Pidgeot, since that was the very first flying Pokémon that I ever got my hands on. Espeon has gotta be my favorite psychic-type Pokémon, and Umbreon is my favorite dark-type. d: Machamp is my favorite fighting Pokémon. With access to No Guard and DynamicPunch, it's able to take down anything that ever gets in its path. My favorite poison-type Pokémon is definitely Toxicroak. It has a very nice-looking design, and it's a great sweeper. And finally, my favorite ghost-type is Gengar. I love its story about being Clefable's shadow, and when it comes to competitive battling, it could make a great special-attacker for any team.
Charizard has and always will be my favorite Fire-type Pokemon. When I first played Blue all those years ago, he was easily one of the most powerful members of my team. Call Charizard an over-rated Pokemon to choose here, but aside from Arcanine or Vulpix, I have not been pleased with the choice of Fire-types that are available.
Water: Oshawott *obviously*
If you had asked me before the fifth Generation, I would have easily said Squirtle. But after the release of Black and White, I fell in love instantly with Oshawott. I have a huge love for Sea otters, and when I got a first glance at Oshawott, I knew instantly that I would love him. His evolution chain's design, in my opinion, was really awesome, and I appreciated his powerful moveset to boot.
I've always liked Treecko and his evolutionary line since third Gen. But after the arrival of Black and White, after my first play-through, watching Cheren's Snivy go through it's stages. I loved the design, and once I got my hands on Black 2, I chose Snivy in a heart beat. I absolutely loved him during that playthrough, and he absolutely wrecked the biggest portion of the Gyms I went through.
I've always been known as a Polar bear due to my abnormal resistance to the cold. I actually embrace cooler weather, and Winter is one of my favorite seasons. Personal aspects asides, I found him immensely useful against Draydon and the Elite Four.
I love dogs, and Scottish culture. So when I first saw that Stoutland was designed to resemble a giant Scottish terrier, a mixture of two things I really like, I instantly enjoyed it. For Black and Black 2, I made sure to have a Stoutland on my team, it's stats were astronomical, and it's moveset was really useful.
Again, this guy looked like a cat or a dog, take your pick. For Gold/Silver, HG/SS, I made sure to use him. He was really good to use against Will, and while his move pool was a little lacking, I made up for it with useful TMs. Overall, a great design and a great Pokemon to use.
Basically, it's the same with Umbreon. I loved the design of the Pokemon, and I actually preferred her moveset over Umbreons. I made sure to have her alongside my Umbreon on my team.
Another first Generation Pokemon for you all. When I first went through Pokemon Blue, I needed something powerful to take down Lt. Surge. On my first visit in Diglett Cave, I met a level 27 Dugtrio that wrecked Surge's team. I was so amazed that I brought him all the way, he successfully ended up beating Gary's Arcanine.
You evolved Onix?! FREAKING SWEET! I regrettably never used Steelix. Definitely thinking about it in the near future. But Steelix's design, along with being the figurehead of the Steel type, I think he's easily one of my favorites.
He's a Fighting/Bug type. Besides his 4x weakness to Flying and Psychic, he really had a nice design, and when I first encountered him during the Bug Catching Competition, I instantly loved him. Sadly I boxed him after beating Jasmine, where I coincidentally first met a Steelix.
He was one of the first Dragon-Types I have ever had the pleasure of having on my team. He fought against both of Lance's with ease, and made them seem like Chumps. In every game that has had Dratini available, I tend to put this monster on my team. I even traded one over to my Pearl version to take Cynthia's Garchomp to the cleaners.
Second favorite would be Zekrom.
This is mostly because I like his design, and his ability PROMOTES Fairy type moves. However, whenever I actually get my hands on a 3DS and X *that's the version I want to get*, I'll definitely try out other Fairy types and make a final decision.
Second favorite would be Gardevoir *again, this is only temporary*.
Not really a big fan of ANY poison types, or the type itself to be perfectly honest. However, I used Scolipede in Black 2 and haven't regretted my decision.
Never could get into Machamp, mostly because I could never trade my Machoke. I like Hitmonlee and Hitmonchan, but something about Sawk and Throh were really significant, even though I came to like Sawk more than Throh.
This was fairly simple, the only other Electric type I can't go without is Pichu. So essentially, I could say either one, as they are both in the same family line. Their designs, along with being so cute and cuddly looking, are really cool. They have pouches to store electricity in them, and their tails are shaped like lightning bolts. I've always enjoyed their movepool and stats above the rest.
Over all favorite....................................................................CHIKORITA!!!!!!!!!
Chikorita was one of the first pokemon I saw and loved it. It was SO CUTE. I like all the other grass pokemon, but no pokemon will EVER replace Chikorita in being the my number 1 favorite pokemon.