Feel free to criticize me, but I think that one of the worst of the worst is...Regigigas. Reasons?
#1: It's gimmicky
For someone like me, who, during Gen III, had no time to do the sidequests to get stuff like the Regis, Regigigas was a bitch. You need all 3 Regis to awaken it, which is especially unfair to people without a DS or DS Lite, as they can't get the stuff up at all. They need to trade with their friends to do that, which, for someone like me with few friends, was impossible. Gen V made it easier, but you still needed friends. Making it the "king of all of the others" was just kinda lame in my opinion; if anything, they should be self-governing. And what kind of king is Normal=type?
#2: Competitive usage is limited
There once was a Pokemon named Single Battle Regigigas. It had its ass handed to it. With the lame ability, it should only be used in Doubles and Triples, and in Singles, it'll be down before the 5 turns are over. In addition, for some reason, Regigigas doesn't even get Rest, which would be amazingly helpful during those first ten minutes.
There are plenty of other reasons, but I can't think of them now. That's just my opinion, but I think it stands.