Baccano
Ouran High School Host Club
Tengen Toppa Gurren Lagann
Speed Grapher
Moyashimon
A few others that didn't quite make it:
Nana - so great that it left a big impression on me when I "marathoned" it in one day even with eye-squinting subs; don't feel the need to watch it more than once
Kaiji - perfect; it's just that I can't watch it again anytime soon `cause I also felt like I lost a lot of money in the end :D
Honey and Clover - I found the lead guy on bike rather melodramatic at first but thank God that's NOT the tone of the story. SHI-NO-BU FTW!!!!!

Samurai Champloo - stylish; loved it that the characters parted in the end; hated the opening theme
Edit: One Piece is my new #2 favorite anime. Rivals DC in my eyes.