I cast my vote for the wheel, for obvious reasons. Without the wheel it would be almost impossible to move large loads. We'd be back to having Beasts of Burden which are highly expensive to maintain in comparison to the work they do. But it's not just limited to that. The wheel makes travel far easier as well. The car, the plane, the bike, the tank, the just about any other major vehicle. Anywho, that's just my two cents.

Also, I have to disagree with those that are saying evolution. For one thing I don't believe it, but thats a diferent discussion altogether. The point is that if it were true then it still wouldn't be an invention of man's. It would simply be a natural process. If evolution counted as an invention, then so would the rain cycle or plate techtonics. And seeing as those are obviously not inventions, niether is evolution.