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 IV solely because of the GS remakes. HGSS are phenomenal games because gen II was so solid. Getting to revisit Kanto after Johto is always so cool. Seeing how it’s changed in the years between RB and GSC. Gen IV also made the best decision to have the special split. They’re pretty much how I remember Gen II being when I played it back in the day, but the quality of life changes modernized the experience. Like the controls actually utilized the DS screen well, being able to run, and having online for battling and trading.
Gen VI is also a very good contestant. Lots of love for the megas and some of the quality of life changes. the music is great and the new pokemon are a lot of fun too. Only issues were with the rivals.ORAS is also solid. I did not care for gen III until ORAS.