Unfortunately this is a hard thing to gage as he is so big and strong, he should never lose, but then that wouldn't be very interesting. As one person already stated he is this generations version of Andre, and that's not really a bad comparrison now, he really can't do quite as much as he use to. And because fans don't want a "real" feal anymore (like when Andre went un-pinned until he let Hogan beat him at Wrestlemania III) they want constant title changes and the unbeleaviable (like Cena giving him the F-U, still impressive as hell) so he's not used the way he should be. But his title accomplishments:
WCW World Heavyweight Championship (2 times)
WCW World Tag Team Championship (3 times)
World War 3 Winner (1996)
World Wrestling Federation / World Wrestling Entertainment
ECW World Heavyweight Championship (1 time)
WWE United States Championship (1 time)
WWF/E Championship (2 times)
WWF Hardcore Championship (3 times)
WWF/E World Tag Team Championship (3 times)
The fact that he still is the only man to hold all 3 brand heavyweight titles, and the fact that other Taker and Kane there really haven't been very big men to stay around...
YES he is a legend.