I'm sorry, I have a new number 1. Not only for what he did achieve, but also for what he would have achieved if he didn't get Lou Gehrig's disease:
JASON BECKER (played in the duo Cacophony when he was 16, got Lou Gehrig's disease when he was 19). There's also a video on Youtube of him playing Yngwie Malmsteens' Black Star in his high school talent show. I think he was only 17!