In no particular order:
Kaitou Kid: Kiddo-sama~
Haibara Ai: Sarcastic humor, tragic past, and tsundere?
Usui Takumi (Kaichou wa Maid-sama: He's just too awesome, especially in his chibi form.
Luffy (One Piece): He's so dumb... But he has a big heart.
Nami (One Piece): Not your average burglar-cat, ne?
Ryuk (Death Note): He's got to be the funniest Shinigami ever.
Sesshiomaru (Inuyasha): He started off as an evil character, and then Rin came.
Rin (Inuyasha): So sweet~
Maou Sadao (Harataku Maou-sama): He's so innocent-looking, that Satan.
EDIT: New favorite character *-*
Levi Ackerman (Attack on Titan/Shingeki no Kyojin): Best. Character. Ever.