Either the bomb-lateral TD play with KG-Pierce-Rondo, the TA block, or Ray's 7th straight 3. The Rondo block on Fisher from behind after biting on the shot fake was amazing too, and instrumental in the win.
As exciting as the Baby-Nate game 4 was, no single plays seemed that amazing to me.
I feel that 2008 had far more iconic plays, with the KG follow dunk, the 2 Ray layups, the Pierce and-1 3 coming back from tweaking the knee, the crazy game 6 KG and one in the lane, and the meaningless cherry-on-top alley-oop to TA. Oh yeah, and the Gatorade.