I don't think any riff can top the opening of Highway to Hell:
http://www.youtube.com/watch?v=sCssYzVl6oQ

Montrose - Rock Candy:
http://www.youtube.com/watch?v=lUeuAnMNDhA

...and for the ultimate 'drilled in to your head' riff, Judas Preist - Livin' After Midnight:
http://www.youtube.com/watch?v=RzpHmbwqHLM