I wasn't being serious about Armstrong.Muhammed Ali was well beaten in several of his later fights, as well as losing the heavyweight title twice. Lance Armstrong was massively improved by performance-enhancing drugs, so I wouldn't put either of them in the mix.
Tom Brady and Don Bradman would be in the conversation. Maybe Jahangir Khan also. Phil Taylor, obviously. Jack Niklaus?
I thought about 'The Power' but couldn't recall his actual name. Brilliant drummer, too, the Philthy Animal!
As a big fan of pitch and put, Arnold Palmer sprang to mind, too.
(Alas, I'm not taking this sufficiently seriously, I suspect).