Paul Pierce diving on a loose ball at the end of Game 7 against Cleveland in 2008, with LeBron looking on. It was the play that tipped the series the Celtics' way, and the perfect microcosm of both Pierce and James.
Pierce did what it took to win that series, whether it was trying to out-duel LeBron on the scoreboard or with floor burns. For Lebron, it showed that despite his prodigious talent, there's something missing in the heart department.
EDIT: Looking back on the box score of this game, Ray Allen had 4 points in 29 minutes on 1-6 shooting.