Duke
The best riff of all time, ever, everybody else put your axes away:
Scuttle Buttin' by Stevie Ray Vaughn
And I can play Smoke On The Water on any instrument you hand me.
Hmm theres other SRV riffs that kick mucho arse over that, like Cold Shot, Texas Flood (but does this classify as having a 'riff' as such?),
Voodoo Child (slight return), no one can top Jimi. No one. There are 20 Hendrix riffs that could easily win a list like this.
Dire Straits are massively under rated for awesome guitar riffs:
Money for Nothing

best classic distortion sound evaar!
Sultans of Swing

best clean guitar evaar! (hmm actually tied with Little Wing, or Castles Made of Sand - Hendrix. Lenny by SRV is good also...)
Romeo and Juliet

the only good use for a dobro national steel guitar evaar!
Crossroads (live 'cream' version) is the best playing/tone Clappo ever did. Only other 'slowhand' notable is Layla (either version).
Smoke on the Water does nothing for me, its just dull and plodding rhythmically. 👎
btw, who can't play smoke on the water on any instrument?
For reference, the most common mistake people make when playing this riff, is play it in fifths (or "power chords"). It is actually all fourths.
If anyone posts "Stairway to Heaven" they deserve to be shot immediately.
