While C.C. is the obvious choice, I'm actually going to vote for Viletta.

C.C. has the smoking hot body and shows it off, but Viletta is often forced into situations where she has to show it off (naked amnesia, Shirely making her do the swim club show, etc), and is objectified by everyone who isn't one of the major characters or otherwise of high school age.

Plus, we saw her bare breasts. And Ougi probably got to touch them.

EDIT: Aww, shit, I should have voted for Milly. Milly in a school uniform is unrivaled. Her skirt seems shorter because she's so tall and has long legs, and she's constantly thrusting her assets out in her standard pose. That, and her VA sounds as sexy as Yukana sometimes.