The best guitar riff of all time?

Tough to say because of the nature of the radio business these days. If they didn't play the same five freakin' songs on every clic rock station every day, it would be easier to compare riffs. As it is, you get so sick of the clics that it is hard to tell. I can't stand Clapton anymore but the first time you hear the opening of "Layla" it is pretty tight, up there with "Purple Haze," "Black Dog," and "Satisfaction." Unfortunately all are so played out that it is hard to be objective. I think I'd give it to Purple Haze or Satisfaction.