I won't bother pointing out which riff I mean, if you know the song you probably already know what I mean. (which is probably hardly anyone on this site, but I'm doing it anyway)
Melvins "Honey Bucket"
Nuclear Assault "Critical Mass"
Living Color "Cult Of Personality" (RClark's first concert reminded me of this epic riff)
Exodus "Verbal Razors"
Annihilator "Allison Hell"
Pestilence "Out Of The Body"
Cannibal Corpse "Hammer Smashed Face"
Deranged "Flesh Rebel" (some of the fastest triplets I've ever heard.)
Morbid Angel "Immortal Rites"
I don't have iTunes in front of me, that's what came to mind at the moment.