I think that the title pretty much explains what this is. Any antoagonist that was just so cruel, or not cruel enough, a crappy villain or too good of one, too manipulative, too soft... I could go on and on. And, if you would like to, give the reasons why. That always makes it more interesting to read.
I'll start.
Naruto: Sasuke. He just has to have somebody to hate and blame for all of his problems. He can't seem to let it go, and it's tearing everybody else apart :(
Inuyasha: Naraku. He's such a manipulative son of a gun, and he just keeps messing with everybody until the very end.
One Piece: Crocodile. I have to say that I hate almost every villain in this show because I love the main characters and don't like the people that hurt them, but I choose Crocodile as my very most hated, because he is the villain of my favorite story arc.
What about you guys?