For me it was Gen IV. Pokémon Pearl was the first game I had physically bought (although I had played gen II and III games on emulators and friends' GBAs before that), which took forever to come out in Europe (we got it 9 months later than Japan). Since I pretty much had to start from scratch, I spent hundreds upon hundreds of hours trying to complete the Pokédex, breeding and training the perfect Pokémon for the Battle Tower, etc., which I carried on doing in Platinum and Heartgold. The anime still seemed fun to watch back then, and had some great openings (at least in Japanese):

https://www.youtube.com/watch?v=bsYrrOLxpSw

The series started going downhill for me when they suddenly decided to rush out Gen V on the DS, which had graphics that would've been better suited for the 3DS. The music wasn't anywhere near as good as the gen IV games, and with the lack of post-game content, I quit playing at around 50-60 hours in. I gave things another chance when Pokémon X and Y out. Although I liked the 3D battling, the overworld simply didn't have the same charm to it, and this was also the first generation to have far too many designs that were genuinely awful. They similarly decided to rush out gen VII, and now they've written themselves into a corner where they can't come up with good Pokémon designs anymore, because money was more important to the creators than longevity.