The one I'd love to hurt the most is Chichiri; thanks to him, I now say "ya know" more often than I really should now. Second is Ren from Chibi Vampire/Karin; I just royally dislike him.
Finally, 17 year old Nanaka from Nanaka 6/17; she's pretentious, she's stuck-up, she's so focused on getting ahead... if she didn't learn how to balance herself out(be childish but mature), I really would have hated the entire series.