All the best guitar riffs were all written by Bo Diddle. Without Bo, there’d be no ROCK in Rock and Roll. My fav would have to “Roadrunner” or “Who do You Love.”
Zillions of answers. Here’s a really unexpected one (at least for me):
The Cars’ Shake it up. Totally lame song; I hated The Cars. I had a job when I was 19 working in a factory, AOR blasting on the radio all day. Foreigner (ick), Cars (ick), Wang Chung (GAH!), (and who TF wrote that piece of shit Tainted Love anyway?? GAH!!) the same damned playlist every day. So you looked for beauty where you could.
But Shake it up has just the coolest blazing little guitar solo in the middle, it’s about maybe 25 seconds long, and it’ll take the paint off of your car in nothing flat. How the heck did The Cars get a wailing guitarist like this guy, and how TF did he stay awake the rest of the time?
And the rest of the answer would be: Bits of ZZ-Top (Billy Gibbons can sure get a Tone going), several Duane Allman riffs (especially One way out where the bassist totally screws things up, but the end of Duane’s solo still gives me a shiver every time), Satriani Surfing with the Alien, etc etc.
The first 3 seconds of Iron Man by Black Sabbath.
The first riff in Holy Wars…The Punishment Due by Megadeth.
Another killer riff which bears searching out is the main riff from “Tempus Fugit” by YES from their underrated album “Drama”
Chris W
“Seek and Destroy” is awesome. Once I learn how to play this, I will consider myself having achieved Guitar Goddess status. Whether anyone else will or not doesn’t matter. 
It’s so hard for me to pick favorites. I love guitar rock, from classic Zep to hair bands of the 80s, to the heavy modern metal of today. I love it all!
“Crazy Train” definitely rates up there among some of my faves.
The other few that I can think of right off the top of my head are kinda slow:
“Over the Hills and Far Away” by Zep
“Love Song” by Tesla
“Wish You Were Here” by Pink Floyd
Okay, I have to stop. I thought of about 50 others while I was typing that. Get me started and I won’t stop.
David Gilmore in Shine on You Crazy Diamond makes the hair stand up on the back of my neck every time.
I know it’s not the proper thread to bring this up, but i am real big on guitar instrumentals and 2, no 3, of my favorites are:
Ronny Montrose’s rendition of Town Without Pity
Les Dudek’s One to Beam Up
Duane Allman’s Little Martha (with Dickie Betts)
Oh, and Robert Fripp’s riff in 21st Century Schizoid Man (for all you old Beaker Street fans) is one of the most furious i’ve ever heard.
The intro to Pleasant Valley Sunday by that simian group from the 60’s.
Neil Young’s OHIO introduction on 4-way street. 15 seconds of bliss.
Muddy Waters and Mannish Boy
As if it could be anything except Whole Lotta Love by LZ. I don’t think anything of the song, but the riff is just the greatest, most enduring riff in electric rock.
I second that one! That extended intro solo is what made me want to learn to play guitar, in fact. That and the guitar work in ‘Brothers in Arms’ by Dire Straits. Although, I don’t think either of those are exactly riffs. . .
But I’m glad to see some Mark Knopfler fans here. I saw him perform on his latest tour, and it was an incredible show. I nominate his intro riff to ‘No Can Do’ from ‘Golden Heart’ as one of the best. Of course, ‘Money for Nothing’ is a classic riff as well, but overplayed on the radio at the time.
I double-checked with my husband, who is a huge fan of Gatton, and the Hellecasters, and he confirmed for me that you are indeed correct, dunne. My bad! 
I totally love the guitar on “Hash Pipe” from Weezer right now. Another one of my faves are Chris Isaak’s “Baby did a Bad, Bad Thing”, which is similar to a lot of Blue’s riffs out there, and I totally adore anything by Bonnie Raitt. Slide guitar is my favorite!
Oh, and there’s that one song from Counting Crows “Hangin Around” that’s very catchy. I’m not really a big fan but I like that song just for the guitar.
Sandalwood, excellent mention of Bonnie Raitt’s slide guitar work. If you haven’t already, check out Chris Rea’s Texas. Some excellent slide work there also, especially when he ‘walks’ the fade-out.
There are so many that I could name:
“Iron Man” - Black Sabbath
“Paranoid” - Black Sabbath
“Heartbreaker” - Led Zeppelin
“Safe as Milk” - Captain Beefheart and his Magic Band
“Sunshine of Your Love” - Cream
“Strange Brew” - Cream
“Jumpin’ Jack Flash” - Rolling Stones
“Day Tripper” - Beatles
“Honky Tonk Women” - Rolling Stones
“Wait Until Tomorrow” - Jimi Hendrix
“Purple Haze” - Jimi Hendrix
“Funk #49” - James Gang
Yep. Check out the IMDB cast page for Crossroads (1986 version). Partway down the page, right under the producer credits, it gives “Original Music By - Ry Cooder” and then farther down under “Other Crew” it lists “Ry Cooder - musician blues guitar”. It’s listed just below the unit publicist designation.
As far as nominating killer riffs, I have to say that I’m partial to the guitar riff that intertwines with the sax riff on Gerry Rafferty’s “Baker Street”
Also the lead on “I Need a Man To Love” from Big Brother & the Holding Company with Janis Joplin. Moderately slow, but compelling in its inevitability.
And the feedback honk that slides into the riff on The Beatles “I Feel Fine” is pretty damn nifty too.
Ah, and Mountain does a damn fine job on the Woodstock version of “Theme from an Imaginary Western.” Other versions don’t have the extra little phrase at the top of the riff like the live version. Very tasty.
AC/DC, Hell’s Bells. Simple and bloody effective. Also, Clapton may have been God-like many years ago, but he’s well and truly past it now.
I’ll 2nd “Hell’s Bells”… I love the dissonance and rythm dynamic created by the peal of the bells… very nice…
Others… “Spirit of Radio” Rush… lotsa Iron Maiden… “the Trooper”, the pizzicotto(sp?) sounding part in the middle of “Phantom of the Opera”, “Wasted Years”, many others… “Stranglehold” Ted Nugent… “Honeysuckle Blue” Drivin’ n’ Cryin’, “Since You Been Gone” Rainbow… “…And Justice For All” Metallica…
And that’s the just the metal that springs to mind… I will pondering this for a LONG time… I LOVE guitar RIFFS!
“Click Click Boom” by Salive rocks too…
“Click Click Boom” by Saliva rocks too…