Edit - I've taken some of your best responses and created this alternative playlist
This came in the post today:
It's all good Friday-afternoon-in-the-office type stuff, and we've passed a happy couple of hours listening to it. It's just that... some of the choices make no sense. A lot of them aren't even riffs. Right off the bat, track one: 'Another One Bites The Dust'. Sure, there's some nice funky fretwork from Brian May, but it's not exactly a guitar shop staple. Think of that song and it's the bass line that comes to mind, not the riff.
Ditto 'Psycho Killer', at the end of CD2. I guess technically you'd have to say it does have a riff, of sorts. But could you hum it to yourself, without hearing the song first?
CD3 is where it really goes wonky. 'Yellow'? 'Creep'? Those aren't riffs. They're chord sequences. What kind of maniac, when called upon to select the most memorable guitar moments ever, thinks: "I know! 'Ruby' by Kaiser Chiefs!" We can make a better list than this, right? Come on, hit me up with your suggestions and I'll turn them into an alternative 'greatest riffs' playlist.