It's between Woods and Federer - both outstanding, how do you decide between them? Okay, then Woods, because golf is such a big 'mind-game', and most traders can appreciate how difficult it is to get on top of those!
Schumacher's problem is that however good he is, ultimately the quality of the car is a big factor. Also his sportsmanship is in question (disqualifications, barging, the Monaco debacle this year), and there seems to be no consensus in his sport that he is the 'greatest ever' - indeed most commentators look back to the 'when men were men' era of Fangio, Moss, etc for the best driver. If you're looking for a current motorsport 'greatest ever' then it has to be Valentino Rossi on the (far more exciting) bikes.