From the BBC
http://www.bbc.co.uk/radio2/chart/guitar-riffs
 
No Black Dog? and Satisfaction seems too far down the list.
 
It looks like the list is manipulated so there is only one song per band.
 
Steve Howe's riff in Close to the edge is still one of my favourites.

This list is bunk. They couldn't come up with a better riff from Sabbath than Paranoid? One Vision by Queen, really? I guess they never heard Fat Bottom Girls or Tie Your Mother Down. We need to make our own list.

Weird collection there.  I'll have to check out some of them later to see if I've heard the riff since a few of the song names didn't click.  Definitely a few missing because of that one song per band restriction.
 
(What, no Low Rider by War?  No Long Cool Woman in a Black Dress by the Hollies?  No Driver's Seat by Sniff & the Tears? No Cars by Gary Numan? No Saturday Night's All Right for Fighting by Elton John? No Hazy Shade of Winter by Simon & Garfunkel? Damn! )
 
Also nothing at all by the Cars, Boston, Foreigner, Styx?
 
And I KNOW I'm missing a bunch - these are just off the top of my head.
 
Just started listening, so on the other side of the coin.  These are my WTF's?

• Status Quo - Down Down
• Manic Street Preachers - Motorcycle Emptiness
• Artic Monkeys - Do I Want To Know
• Pearl Jam - Alive
• Queen - One Vision (you have to be kidding me with all the others they have!)
• Sex Pistols - Pretty Vacant
• Radiohead - No Surprises
• Oasis - Cigarettes & Alcohol
• The La's - There She Goes
• Edwyn Collins - A Girl Like You
• The Cure - Boys Don't Cry (really?  Love Song and Fascination Street blow that one away)
• Rainbow - Long Live Rock & Roll (definitely better ones like Man on the Silver Mountain)
• Bo Diddley - Bo Diddley
• Buddy Holly - Words of Love
• Isley Brothers - That Lady
• ELO - Ma-Ma-Ma-Belle (just pick about any other song from their Greatest Hits)
• Eddie Cochran - C'Mon Everybody
• Foo Fighters - Wheels
• Dandy Warhols - Bohemian Like You
• Primal Scream - Rocks

I give up.  The list is almost completely crap.  I'm guessing the person who put it together only went through his own collection and doesn't really know what a guitar riff is!  Some of those on the list were just a few chords - not a riff at all!

WHERE THE HELL IS THE LONG REPLY I JUST MADE??!!!!!!!!!!!!! 

Only 1 Beatle song is kind of odd as well. Birthday, I Feel Fine, Ticket To Ride are several that come quickly to mind.

Here's my list:
 
Symptom of the Universe.
 
It occupies all 100 top postions.
 

More seriously, I think that those lists are the easiest thing to generate clicks.
That's pretty much the only purpose they serve.

*Click!*

I think the people who put these lists together do so for the sole purpose of watching how many people get upset since it is impossible to match everyone's list on something so subjective.  The formula seems obvious in my opinion on all of these "greatest ever" lists.  1) Grab about 75 popular rock songs. 2) Throw in a few blues songs and a few token blacks to defer criticism of the obvious bias towards one genre 3) Toss in a few recent chart toppers just to get purists extra pissed 4) laugh maniacally as you patrol internet to laugh ta the "outrage"
 
I'm not saying this is a great list or would even be my picks, but it does feel like they are trying to see who gets rattled. I'm not even sure wha tthe criteria is.  I suppose each of these songs could make a case depdning on the criteria. The guitar riff in Mannish Boy is not the first use of that riff and arguably not the best version from a technical point.  At the same time, I probably think of that song first when I hear that riff. "Beat It" may not be Eddie Van Halen's best solo but is without a doubt the song he played on that had the most gross sales.
 
I think rain summed it up best : These lists generate clicks regardless of what they contain and generate threads like this all over the web.  Good business since there is no scientific way to prove any of these lists right or wrong.

AFAIK, the list was compiled by asking listeners of the radio station for their favourite riff.