In reply to Deadeye:
Why not?
I mean, even the greatest tennis player of all time is a great debate, and a hard call. Is he better than Laver- two grand slams? What if Borg, McEnroe, Connors and Lendl hadn't all been playing roughly the same time- if one of them had had a clear field the way federer had in the early part of his career, how many slams would they have one. Interest that Borg still holds the best slams entered: won ratio.
In the end I think he is- 19 majors, longest time as world number one, his utter domination when he was at his best, and remarkable longevity, make a stronger case for federer than for any of the others.
Greatest sportsman of all time? Well, comparing sports is so hard that it's bound to be just speculation.
But fun speculation all the same...
You could make a 'short list' of sportsmen who have been so dominant that they transcended their sport- bradman from cricket, pele from football, bolt from athletics, Nicklaus/woods from golf, Ali from boxing, phelps from swimming, Armstrong from cycling (oops, maybe not... ); if you did, then federer would have to be in there. How you pick the best from that lot, I have no idea, but his dominence of one of the few truly global sports gives him a decent shout I'd reckon...