IMO, Tiger is THE standout athlete, sportsman, whatever you want to call him, in the world. Probably ever. Not just compared to tennis players, but compared to just about any sport you can name.
A few comments, not necessarily in any order but just based upon stuff I have read in this thread....
* He has no serious competition from his peers? That's because he's so bloody good. There are plenty of great golfers going around, but Tiger just crushes them.
* Federer, whilst in his twilight years, just can't beat Rafa on clay - in fact if you look at last year's French Open, he pretty much laid down and gave it to Rafa. Ever see Tiger lay down on Sunday afternoon and give away a Major?
* When the going got tough at Wimbledon last year and in Melbourne this year, Rafa beat Roger. I can't imagine Tiger losing 3 in a row to Mickelson, head to head.
* Usual stuff - to win a tennis Grand Slam, you need to win 8 matches and probably only 2 against top 10 players. You can play average in the first 4 rounds and still scrape through, as Roger has done plenty of times in recent years. To win a Golf Major, you have to play 72 holes against everyone in the top 150 or so players. Almost zero margin of error compared to Tennis.
* Winning a tennis match is a bit like playing Stableford, or even a Par event. If you have a shocker game, lose your serve to love, so what? You just break back next game. If you make a triple or quadruple bogey in a stroke event, it could take a full round to make that up again, and then it might be too late. Look at Bethpage last week, Tiger got a dud draw with regards to the weather, had a poor final four holes in the first round and that was the difference between winning and losing the US Open. To me, this is the equivalent of losing a service game in the first set of a tennis match.
Yes, I am biased towards Golf and Tiger, but anyway you want to measure it, I think Tiger's record will stack up against any sportsman.