Ooh, venting!
I one I hate most is unquestionably Tooya from Ayashi no Ceres. I hate his oh-so-perfect, utterly lacking in personality, overly powerful GUTS. Also his stupid girlfriend (Aya, that is). I also despise Miaka and Tamahome from Fushigi Yuugi.
...Really, I just hate Yuu Watase's manga. I swear, I open a volume from one of her series, and I come to hate the entire cast within the first chapter.
As for characters not written by that woman...Kaguya from Planet Ladder is the only one I really, entirely hated in all respects. Something about the way she would call people selfish because they weren't always thinking only about her and her comfort. The hypocrisy, it burns.