Max destroying the Crystal of Souls, thanks to everything he learned from the heroes.
To some extent, anytime Max works up the courage to face Skullmaster is pretty awesome, because Skullmaster is just that damn frightening.
At the very end of the series, Skullmaster has killed all of Max's allies, and all but succeeded in conquering the world and controlling time itself. Max, a young kid, finds himself all alone and without hope. He has nothing left, and every reason to give up...but instead, knowing that he's almost certainly going to die in the process, he gets up, and hurls himself onto Skullmaster's staff in a desperate attempt to stop him. Yet, against all odds, (and with the series' action theme playing) Max succeeds. That's awesome in and of itself, but what really hits home is Max's Badass Boast.