I try to tread lightly around this, because I know Pierce is a fan favorite, but he really did not outplay LeBron. LeBron was guarding Pierce basically 1-on-1, the entire Celts D was geared on LeBron with heavy amounts of direct help from KG and Perk to prevent LeBron from just driving past Pierce. And even with that, LeBron still put up better numbers than Pierce and affected the games in a larger way than Pierce. It just so happens that KG and the rest of the Celts were much better than the other Cavs, which is why the Celts won, but Pierce wasn't the best player in that series. In fact, he really wasn't in the top-2 overall across the entire 7 games as those honors belonged to LeBron and KG.
I'd say LeBron is probably the scariest player in a 7-game series, but I put Duncan and KG right there with him. LeBron's combo of offensive skill and size makes him a playoffs nightmare, and as he showed against Pierce his defense is coming around as well. Duncan and KG are the best defensive anchors in the league, which is critical in the playoffs, and each also tend to lead their teams in scoring and impact every other aspect of the game as well.