I nominate "Rock Bottom"...UFO. Just a great, great riff.
How about "Do you feel like we do", not sure if that is the real name of the song from Peter Frampton but I can go for days battling to get that riff out of my head.
"Stayin' Alive" Bee Gees. Hey just as much rock as "Beat It" and a nice riff. In my opinion the bass line from "Billie Jean" is one of the best there is but it is not a guitar riff.