Gotta be Satchel. Any player that can call all his infielders to the mound, then have them wait there while he strikes out the side can take the ball for me any day. I know there weren't the greatest of stats in the Negro Leagues, but it's rumored that he won something like 2,000 games there, and when he went to MLB in '48 he was sick there, too.
In seven seasons in MLB, he put up a composite 3.23 ERA (take out one season and it drops to about 3.0), which doesn't seem amazing until you realize that he was aged 41, 42, 43, 44, 45, 46, and 58 in those 7 seasons. Wow. Imagine what he could've done in his twenties.