1. Roger Federer
2. Marat Safin
3. Andy Roddick
These are the players that when they're AT THEIR BEST, are untouchable by the rest of the tour. They have all proven on their worst surface - clay.
And have won hardcourt slams. And are forces on grass.
Of course, Roger is the peak player of out the three and then Marat and then Andy. They have dominated recently and will continue to do so in the future.
In fact, these three have won 8 out of the last 10 grand slams. The other two won by claycourt clones. It's only a matter of time before these three catch up and dominate the clay too.