Gen IV I think. I had just started College and was free to like whatever I pleased without being the immediate target of a hate mob, and Gen IV introduced so many new features and Pokemon to the franchise that made the games so much more appealing than they had been during Gen III. I was more active on the internet during this time so I could watch episodes online and catch up on what I'd missed/keep up as it aired, and since the games introduced online I could play with my friends as well.
The Pokemon themselves were also fantastic - most of my favourite Pokemon come from Gen IV - and we also got HeartGold/SoulSilver to cap it all off, which are (in my opinion at least) the gold standard for remakes of any kind and still the best Pokemon games ever made. Gen IV felt like a perfect coming together of everything I used to love about Pokemon with everything I now loved about video games, and it was just a really good time to be into the franchise.
Gen 4-5. I remember I had Pokemon Diamond, SoulSilver and White 2 when I was younger and playing them to death. I would always talk to every person, explore every tile and would often open the map and speculate on what the next town/routes would be like and what cool Pokemon I'd find. I had a few other DS/3DS games like The Legendary Starfy and Mario Kart 7; but those games were always my go-to. Even taking out the nostalgia, they're all still excellent experiences with tons of content; great post-games and amazing spinoffs like PMD: Explorers of Time/Darkness and the Ranger games.
2: Still carried the general excitement from Generation 1. Liked Gold and Crystal more than Red and Yellow. No longer shared a game too.
6: Got into the Battle Tree this generation. Never cared much for singles. Found it fun to use various doubles strategies that never came up in the main game.
Also started watching Nuzlockes around this time. Participated in some Wonder Trades with the player. Enjoyed the new way of approaching the game (especially after breezing through X/Y with the Experience Share) and figuring out good Pokemon to send at certain points (or in general). Reinvigorated interest in the series.
7 was the point where I knew a lot of people I was actively playing with at the time, and I think the games were very solid. I had a lot of fun and felt really engaged with the series. During Gen 4 I was playing with a lot of friends at school, but 7 was at a point in my life where I knew more people and could appreciate that a lot more. Not to mention I understood online play a lot better.
I'd actually say that Pokemon is at its best for me right now. When I was a kid, while I was super obsessed with it around Gen 4, I never really built good teams or had that many people to talk about Pokemon with until I got back into the series. And I didn't know how to emulate, and couldn't replay my favorite games on real hardware because I didn't want to overwrite my save files.Today, that's all different. I can play Pokemon games as many times as I want and create as many save files as I want, and I found this wonderful website to post about them on.
Nothing will ever beat the highs of beating the Gen 3/4 champions for the first time, but the overall experience of playing Pokemon is much better now, and I can't get enough of it.
As far as appealing gameplay goes and me spending lots of time in the game without getting bored easily, I really have no choice but to answer with gen 8. It's a good game.
Good selection of mons, no stupidly big PokeDex to complete. Prior to Shield I don't think I've ever bothered trying to complete one.
Mechanics wise it's easily my favorite. There were several QoL mechanics that didn't exist before, such as Box Acces from anywhere. The changes to the breeding mechanics actually got me to pick up breeding for the first time in the series. I never bothered with breeding prior to Sword/Shield. I also never shiny hunted prior to Sword/Shield (though I did return to USUM for the Ultra Wormhole shinies at some point after completing Shield).
Raid Dens were nice, but didn't really hook me that much. Mainly used them trying to get HA mons. Dynamax Adventures on the other hand was very fun and I've sunk a lot of time into them. I still occasionally go on them just because I find them fun.
As far as gyms go I find Sword/Shield's gyms the most enjoyable of any game's gyms. They were turned into a sporting match of sorts and Dynamax is the generational gimmick I enjoyed the most as it actually felt necessary to use it in battle, unlike the others.
No other Pokemon game comes even remotely close in terms of time spent in it to my copy of Shield. Shiny breeding is definitely the main cause for this lol
White 2, to this day, is the game i put the most time into with all of its content. It was definitely peak, the DS games in general i believe were peak. After that the games started getting slowly worse imo. Not immediate, every mainline game even at worse id give like a 6.5 or 7. But i definitely remember picking up Y, finishing it, and asking "Wait thats it? Wheres the other cool stuff". Nowadays it seems most main games have a single cool thing in them, whereas before games had MANY cool things.
2009-2012 for me. i was already deep into it after having been such a huge fan of DPPt & HGSS and seeing just how amazing BW/B2W2 were (and still is), i figured it couldn't have gotten better than that.
Definitely gen 4-5. Gen 4, I could finally play with online friends but I still had a couple irl friends who played. I prefer DPPT over HGSS, but HGSS was basically the perfect remake--I don't know a single person at the time who didn't like what they did with gen 2, which was generally the big fave at the time. But even beyond the games, I think Pokémon in general was just in a good place around that era. There was still a lot of interest in it online but it was concentrated on forums like PC because everywhere was more decentralized, so you'd get these great pockets of community instead of everyone just using reddit or discord and never really sticking around past when they need something. It was also when you'd get a lot more merch at least in North America. You can find plushies and stuff again now, but it's mostly just Pikachu and we went through quite a few years where sometimes you'd go into a toy store/aisle and not see any Pokémon stuff. Also I think for me, Pokémon designs peaked in gen 4 and 5. There are just so many good ones in so many different styles.
Gen 4/5 imo. I was still a kid and this is where I really started getting into the series on my own outside of just being into it because my older siblings liked it.
Got a lotta good memories with them, being completely blown away by the atmosphere of Black, being completely confused as to how the Entralink worked, feeling adventurous going through the Distortion world and defeating Cyrus, having tons of fun in the Wi-FI plazas, messing around with silly Action Replay codes, etc etc
I kinda regret not using the Dream World while it was still up. I didnt really have internet access over the week and I didn't really have my own computer to use it with, so I never really figured that stuff out. I hope people make a recreation service on Kaeru WFC at some point, though that sounds really difficult lol