i must say I am quite pleased that Sabbath is dominating Zeppelin on this thread, as they rightfully should by the way, but y'all all know I got Jerry G on the brain when it comes to guitar licks
also, here is one of the all-time most popular/mainstream and yet powerful of reggae guitar riffs:
http://www.youtube.com/watch?v=YDMxLbMcjxY] the OG concrete jungle