I'd say Rondo and Jeff, but I like them all, they're all Celtics until they show otherwise.
One guy I'm intrigued by is Faverani, I didn't see him as a special player in the ACB League in Spain, but his stats were really good, and he could be a nice surprise if he shows on the court what he's been preaching: toughness, effort, defense.
I think every guy on the roster has a chance to grow and contribute, even Bogans who's a veteran and could be useful as a mentor, as a player or as a valuable expiring contract.
I can't wait to watch them play...