I honestly gotta go with 2, it really upped the ante from the first generation and introduced a lot of cool mechanics. Being able to go back to Kanto and eventually fight Red also *blew my mind* at the time.

I don't think this is just nostalgia because I replayed Pokemon Gold - the original, not HeartGold - last year and it hasn't aged a day. Played through the entire thing in like a week, loved every minute of it.

In second place I think I gotta go with XY, Pokemon is great these days.