In terms of who is the greatest in any sport, I'd agree with those who say it's very difficult to compare different players from different eras.
One thing that I think does separate certain individuals is whether they've actually contributed to major changes in their sport. For example, in snooker, Stephen Hendry pretty much paved the way for the modern game by making long potting something to be attempted rather than avoided. Until Hendry came on the scene, snooker was very safety led and then he arrived taking on and potting anything in sight. Does that make him better than O'Sullivan? Maybe not but I think it should be a big factor. You can also say the same for Tiger; he pretty much led the way in aggressive, attacking, long-driving golf. Again, does it make him better than Nicklaus? Debatable but a consideration.