Sawako from Kimi ni Todoke, the world would be a better place with more people like her around. EDIT: Actually, Shana from Shakugan no Shana may be my favorite... It's a tie between the two.
These all almost always stole the scenes they were in: Spoiler Diva from Blood +. Rena Ryuugu and Mion/Shion Sonozaki from Higurashi. Haruko from FLCL.