Someone mentioned Smoke on the Water. There are lots of other great riffs from Deep Purple's Machine Head as well .. Another I like is the riff from "Maybe I'm a Leo".
Kickass song and nice example of what I think of as a riff.
edit :
To JH, those songs and riffs are all familiar except that Judas Priest song. I don't have that album, but that track sounds great
Definetly gonna get that one.