As far as specific plays, my two favorite of the year were Rondo's dunk in the Detroit game and the Ray Allen up and under layup against the Lakers (especially cool in the slow motion video shown on the first page of this thread).
But as far as a general play, I love when Ray Allen is spotting up for a three. Everyone in the Garden gets on the edge of their seat as he catches it, ready to explode when it goes in. As he was breaking out of the slump in the playoffs, this electricity was about 3x what it normally was. It is really fun to experience.