@Waldbrand gen 3 was the first one I played, including collosseum. R/S/E are in that sweet spot when Pokémon still had that 2d charm, and the developers could realize a lot of what they wanted to do with a Pokémon game, but it hadn't gotten to the seeds of the current state of Pokémon. There's enough Pokémon where it doesn't feel like they're reaching for designs, and they're all very memorable. It's also before any crazy mechanics like the mega evolutions. It's just a simple Pokémon game at its best. The music is amazing and the post game content is pretty good.

I think Sun/Moon's generation would be a close second, kind of for the same reason of it perfecting the new 3D Pokémon style before it all went to shit.