It's hard to pin down just one, but I think a notable mention would be run-dmc.
They melded rap/hiphop/rock together so well and uniqely that there has been noone else since to even come close to any sort of emulation or style.
It's funny though, I can't stand rap (postering fools), or hiphop (candy arsed bubblegum crap), I do like rock. But those guys put it together so well and when they collaberated with other artists such as aerosmith, it became legendary.