If it is for singing alone we give this honor then I would vote for Elvis. The Man had an incredible voice which he could and did apply to a number of different genres of music, and with which he did broaden the view of what great singing could be- what vocal styles could be acceptable. That is a lot to do in one century! Still, my favorite bests are the likes of Woody Gutherie and Bob dylan. Captains of the best worst male singers of the twentieth Century catagory to show up here, er, eventually.
