Marquis first for me because I was really skeptical about him, I thought he was "Tony Allen 2.0" but he is clearly the bench player who has surprised me the most, and I love his game and his IQ. He is the core of our 2nd unit and even though his stats don't show it, he is the most important player of the bench. In one player, we've got all we need (ball handling, good to great D, passing game, good vision, and effective low post and midrange game) for the second unit, expect the shooting touch.
Second, I will have to go with Rasheed. He is in a slump right now but still, he was targeted by many teams and Danny (and Doc + the Big Three) managed to convince him to come to Boston. I'm not worried about his shooting struggles, he will be a big help for us during the season and the playoffs.
And third, Shelden! Didn't expect anything of him and he has been the most consistent Celtic out there. Hopefully he will be able to contribute the same way even if Baby "steals" him his minutes when he returns.