Aw, so much dislike for Gen 5 ;_;.....Generation V was my favorite.
But, uh, as for my least favorite Pokemon game......I guess Red/Blue/Yellow is up there. Yes, I know that without those we wouldn't be where we are today and that they were good for the time; I had fun with them when I was a kid. But it's just not something I'd wanna go back to. Red/Blue/Yellow is a horribly unbalanced, technologically inferior thing. And the post game is ass (all it consists of is "catch Mewtwo", nothing more). But I suppose I can't hate on it too much, it's not it's fault that it's old and was the first games (and there fore kinda experimental).
But another set I'm not that fond of is Ruby/Sapphire. The story and its evil teams were just one of the worst in the series for me. None of the characters were all that great, and worst of all, the post-game was ass like in R/B/Y.
Now, don't get we wrong, I don't hate these games, it's just that I didn't think that they were that great (which is kinda funny since the majority of the fanbase seems to think that R/B/Y, R/S/E, and FR/LG were the best things in the series).