It changes constantly, but this my current favorite ZZ Top riff:
http://www.youtube.com/watch?v=nKueHdKJ0jg
Pull on those strings with chip-on-shoulder ‘tude.
Gotta love that thundering bass rolling underneath the whole thing, as well.
Never knew this one until college... that was when I started getting into their older stuff
“Long Distance Boogie” from the Fandango album — recorded live, hot and righteous~!