I love all the Pokémon games, but my favorite is probably either Gen 1 remakes in Gen 3, Gen 5, or Gen 2 remakes.
They’re all really good though, so I’m not sure I have a definitive favorite. I guess they did get a bit lackluster starting with Gen 6, though that one, 7 and 8 are good too. I think Gen 7’s only issue imo was the amount of short battles even in later routes, so many 1 or 2 Pokémon battles for a good while. Gen 8 was very easy, but I may have overleveled my Pokémon a bit, though I guess overleveling is easy to do in Gen 8 too, heh; still cool though, and Dynamax Vespiquen in Isle of Armor beat me a time or two.
I don’t remember how Gen 6 felt tbh, I need to play it again sometime. It may have been a bit easy, idk, but I remember still loving it and having fun as I usually do with Pokémon, loving the franchise since I was 3 or 4.