He has kryptonite and his name is Mark Selby. Though it also depends on the mood Ronnie is in. When he's not feeling it, then lots can beat him. But when he's on, no one can.
His long term rivals were Hendry, William and Higgins. Hendry has retired and the other two like Ronnie are getting older and not has great as they once were.
But it's the consistency that Ronnie has that sets him apart (apart from his natural talent)
Hendry wasn't really a rival so much, as he was coming to the end of his era. Hendry had been around for a while, then there was Higgins, O'sullivan, and Williams. If my memory serves me correctly. Selby came a long a bit later - he knocked on the door for a bit, until he won he's first major.
It's O'Sulivan's longevity, that's the most impressive, for me. Winning he first major at 17, and still winning them now. Of course, it helps that all the other greats are on the wane too, and the younger guys just aren't as good.
Davis - Hendry - O'Sulivan, Williams, Higgins - Selby (the last great player to come a long).