For my money, the riff of this past decade; (and sadly, yes, I am the kind of person that will take/waste the time to determine things like 'favorite riff of the decade/year/album' and throw that information into a mental pile with a bunch of other useless shit;) belonged to the White Stripes for, of course, 'Seven Nation Army.' I know the riff is so popular (and overplayed perhaps) that it's much more fashionable to bash it, or perhaps, cite some much more obscure chord progression, but that wouldn't be honest to my reaction when I first heard the song. (And, for that matter, the 100th time I heard the song.) When I first heard it, I didn't even think it was a guitar riff at all. It sounded like a bass line, which was surprising, because the White Stripes don't have a bass player. Rolling Stone.com has an interview with Jack White where he discusses the riffs inception. Apparently the riff came to him in one piece, during a soundcheck. Interestingly, White played the riff for an employee at Third Man Records who was on the tour and at the soundcheck. His original verdict... "it's OK." This amused White.
What I find amusing about his recollections of the birth of 'Seven Nation Army' is that the riff came to him, in it's entirety, rather then being something he slowly worked at. I'm sure this sort of spontaneous stroke of genius often comes to musicians, but what's funny about this is how unusual this riff really is. In order to achieve that bass-like sound, White is running his semi-acoustic guitar through a whammy pedal set down an octave. You know, the sort of thing you casually do while tinkering around in a soundcheck! It all sounds like such a specific series of choices, clearly designed to make an unusual sound... and yet, he does all this then stumbles upon an iconic rock riff spontaneously.