I know people hate Jiggs, but he's pretty unique, and excecuting the perfect WoP feels great.
I hate Diddy. I wasn't too impressed when he was revealed. He feels a little jerky to play with (though, I hate to admit it, a little satisfying to play as). However, small, fast characters such as Squirtle and Diddy are hell for my Link. Ever since his inclution in Brawl, I've started to develop an irrational hatred for him, even quitting and restarting the match if the random function pits me against him, or even making me target him if he pops up online on Mario Kart!
Sorry, Diddy-lovers...
Oh yeah, Falco too. Him and his bread...
I suppose MG&W, but that's most likey irrational since, in Melee, me and my friend were under the childish expectation that the last character you unlock would be Master Hand. Hey, we were young! So, we battled for hours to get to the last character, and who turned up?
So, I guess there's just been a negative association in my head with G&W. It's not helped by the way he's so good... *Grumble*