For me, it's definitely Hollow Ichigo (not Ichigo with the Hollow mask on, the one that takes over whenever he's about to die, like when fighting Byakuya in the Soul Society arc). I especially love it when he laughs, it's just so perfectly crazy and evil.
Second place is a tie between Kenpachi, Uryu, and Kon. Yes, Kon; I've always hated the fact that he was reduced to mere comedic relief from the very episode after his appearance, even if that comedic relief is part of the reason why I love him.