In terms of which gen is the most fun for me, it has to be gen 2. Johto’s my favorite region, the starters are my favorite along with so many of the other mons introduced, and Stadium 2 offers the best holistic experience for me out of any game in the series. The Stadium/Colosseum models are so much more full of life than the current ones, though admittedly they had a lot less Pokémon to account for.
That being said, I also would be remiss not to acknowledge that the battle system is so much better now than it was back then, particularly with the Physical/Special split and the addition of Fairy type. That’s why I’m actually really hoping for a Let’s Go Johto game to come out, since HGSS added the former but not the latter, and it’s hard to go back.