I think two for me. In my opinion, it had the best animated sprites (later animated sprites were sometimes the entire sprite bouncing which felt lazy to me). I stopped playing after gen four though, so I’m definitely missing some.

Whichever generation was in the Pokémon game for the original Gameboy color. The one where there was a red, green, and yellow version of the same game.
That’s the only real relationship I have with pokemon, because it was such a great game. So great that I loved the game and played it on an emulator, as I didn’t have a Gameboy.

I haven’t played every generation but imo generation V (Pokemon Black/White) were the most interesting to me. It’s the sweet spot between the older clunkier games and the more modern ones.

Gen 2 was best, because of that miltank that they nerfed in later versions.

Gen 2, Crystal. Was my favourite. I found the character selection and graphical upgrade was really nice. Gen 2 includes the previous game world. Collecting all the badges again was special.

But I played Gen 1, Blue the most. The missingno trick was a bunch of fun and it was my first gameboy game.

Edit: my friend said Soul Silver is peak and sun/moon is a close 2nd place.