Who would win a game of one-on-one basketball?
With the exception of the Agassi matches, I haven't watched men's tennis in years. The new crop of highly-ranked players lacks personality. I might recognize Roger Federer if I saw him at the grocery store, but I couldn't even name 5 other guys in the top 15 in the world right now. I assume Nadal and Roddick? That's about it.
I don't doubt that Roger Federer is a great player, but the fact that he's rarely - if ever - challenged means that his contemporaries aren't good enough to create compelling match-ups.
In golf, though, Tiger is quite often pushed to the brink and he loses more than he wins. I mean, when you've got 140 guys gunning for you week in, week out, you're going to lose (as in, "not place first") most of the time. Maybe if every event was match play (which would get boring after a while), we'd see different results, but part of the fun is the chase.
I'm not trying to hate on Federer, I just don't agree with the claim that he's the greatest athlete - or even tennis player - ever. Sure, it's not his fault that no one else is good enough to challenge him, but the sport needs that in order to be entertaining.