I bet there is a character we all hate, a character that we suck with, or a character that we just hate it when someone plays with it.
For me, I don't like much when my friends choose Kirby, because they only eat you, and suicide with you. Oh yes, and the annoying Down+B thing.
For the characters I suck with, I try to learn to play with everyone, but I suck with Mario, Sonic, and a couple of others