Don't know about the best dunker, but the dunk of the year was Rondo against the Pistons. Blew by Rip like he wasn't even there. To me, that was one of the definining plays of the season; that was the moment when it was clear, the Pistons couldn't deal with his speed, and the C's were just the better team.
Seeing Powe get up and throw it down was really encouranging, too. He's got a lot more athleticism than people thought he had left after his surguries.