Ignoring remakes:
Gen 2
Gen 5
Easily the best two, followed by:
Gen 3
Gen 1
In Gen 1's defense, it really hasn't aged well.
The rest were just too damn boring. Didn't even finish Y (7 badges) nor Moon (I don't know, my all bird team had levels in the high 30s I think). Diamond was so boring that I barely remember what happened in the game.
4 ≈ One