The message board post I'm quoting below can be found here on the Something Awful forums. The person who compiled the list has a blog called Strange, Weird Thoughts and while I don't necessarily agree or disagree with the content of the list, you can't help but applaud the time and effort that went into it. At the very least, it's worth looking through the list for a bunch of the YouTube videos that are included.
A few months ago, I was bored and I decided to make one of those "best
of" music lists that sites, shows, and magazines make all the time. So,
flash-forward a few months later, I have compiled a list of what I
think are the 500 greatest guitar riffs of all time. I've posted a link
to each set of twenty that's on my blog. For most of the songs, I've
linked a YouTube video (audio quality may vary for each) and a decent
guitar tab for the songs. Apologies for the large amount of anime
videos that I ended up linking to, often they were the only ones that
weren't a lovely live performance.
My process for determining which riff is better than other riffs is
that I basically thought to myself, "Hmm, does Song A have a cooler
riff than Song B? Yes!" However, I also took into account the riff's
influence, how catchy it is, and how fun it is to play on the guitar.
I'm posting this list here because music lists usually generate good
conversation on GBS.
I obviously have never heard EVERY rock song ever made, but I've heard a lot.
500-481
480-461
460-441
440-421
420-401
400-381
380-361
360-341
340-321
320-301
300-281
280-261