There are so damn many awesome guitar riffs out there, but I am not going to mention specific songs, but bands that deliver killer riffs.
Iced Earth
Children of Bodom
Rhapsody
Symphony X
Primal Fear
Psychotic Waltz
Racer X (Paul Gilbert is the guitar god)
Vanden Plas
Silent Force
Gamma Ray
...the list goes on forever, but those mainstream riffs, or what I call mainstream riffs get old very fast. But, for mainstream metal riffs, I would say the best would have to be Megadeth, rust in peace, other than that, there is not really anything else mainstream that catches me. I am not a big Megadeth fan, just happen to like that one cd, but by far not nearly as much as the aforementioned bands in the above list.