my favorite was gen II, but i'm biased there because it was my first pokemon game
as for worst...i didn't dislike any of them, but i feel like x/y has less replay value than the others. i've been replaying a lot of the games lately and Y was by far the least fun.
edit: and i also don't like that it has basically no postgame, and i'm thankful that they added something in ORAS
