I wasn't going to get involved in this riff-off going on between Carl, Simon, Matt, Seb et al. The general 70s hairiness of its first few rounds was interesting though. I was trying to decide whether this was because of some kind of implicit set of rules which could never be spoken aloud or even acknowledged, like Mornington Crescent or something. If so, I propose that Rule 2 for this and any future outbreaks of Riff Swap should be that anyone using one of Led Zeppelin's is automatically disqualified.
Sticking to the hairy and pre-punk, I would nominate Creedence Clearwater Revival. I think Suzie Q is a pretty pure example of the form, simply because of the way the first three notes set up and demand the subsequent pay-off of the notes which follow. Born on the Bayou is just gloriously economical, the riff stripped down to these three notes hanging in a laconic lip-curl over the rest of the sound.
Carl mentions hiphop here in the context of rap-rock crossovers. I'd been thinking about hiphop during an earlier round, because isn't it the case that hiphop, at least while sampling ruled the roost, was deeply implicated in the Riff economy? What else is a classic loop than the recognition, cutting and pasting of, a riff (on guitars or otherwise). I know this is stretching the term, a lot, but it's probably worth admitting that really the Riff is essentially just the Hook, renamed for the rockist palate. Acknowledging the close relationship between the Riff and hiphop fits with Matt's developing project on Dissensus and Cybore (in which he is becoming the Northrop Frye of music writing), as it allows hiphop to slot happily into the Rock genus. [I've got more to say about Matt's set of master genres when I get the time.]
Note also that hiphop is capable of maximizing the latent Riff potential in the flute, which, as noted in Simon's bit on Kraftwerk's Ruckzuck for The Wire's Riffs feature, is not easily Riffed upon.
You'd expect some flute within the backpack tendency, it does tend to connote mellow/jazzy/tasteful/yawn etc:
But you also find it at harsher grades:
Souls of Mischief (from 0:23)
And there's a whole strain of hiphop flute which is just pure filth.
Dre & Knocturnal
DJ Quik & Kurupt
P0rn flute! What's going on here? Something like the pimp logic of deploying 'effeminate' signifiers as an assertion of super alpha maledom perhaps.