Personally i’d say XY and gen 6 in general, mega evolutions are excellent and they made an actual effort putting lore into the games (plus i’m french so i may be biased because kalos), ORAS were imo the best remakes ever and are exactly what a remake should aim to be.
Gen I. For some reason it was just the one I kept coming back to, maybe because it’s so easy to hack. I loved Pokemon Yellow so much. Gen II having both is nearly a tie though, and if hacks didn’t exist would probably be my fav.
But Gen III had the absolute best game in terms of ‘just right difficulty’, Pokemon Colosseum.