I'm shocked no one has mention James Hetfield. The man basically was (is?) Metallica. He wrote almost every riff they have and to play underneath Kirk Hammett is simply outstanding given how complex some of their time sigs and head banging can be.
Also, I'd like to give honorable mention to Adrian Smith of Iron Maiden. He's the only Maiden member who doesn't notoriously shred away and the man can carry the band for measures upon measures, with some of their songs lasting as long as 9 minutes.
I was looking for this one. James Hetfield is hands down the greatest rythm player of all time. Even if you don't like Metallica or know anything about them, this guy is worth checking out. He was my inspiration to pick up the guitar in the first place so my opinion is obviously biased. So check him out for yourself. His playing is tight, versitile, and always spot on.