And “Black Night” has a better Blackmore riff than “Burn”.
The first riff I thought of when I read the thread title was Whole Lotta Love. To me, that’s the quintessential rock riff.
There are so many it’s hard to narrow it down to one.From Jeff Beck to VanHalen. One of the most well put together and smoothest that comes to my mind is one from Pablo Cruise, Love will find a way. Very well put together, not too harsh for those who don’t enjoy guitar, and easy to reproduce (with a little practice). Never fails to garner a smile from those unfortunate enough to hear me play.
http://www.amazon.com/Love-Will-Find-A-Way/dp/B001CJO0UO/ref=dm_ap_trk1
I tried the link and it doesn’t go far enough in the tune to hear the solo. Que sera, sera.
How could this have went a whole page without anyone suggesting Eruption?
Got you covered. As someone who heard this when it first came out, it blew me right the fuck away. I had never, and I mean never, heard any thing like that shit . What’s the word for it? Watershed? It meant a whole new ream of equipment for me and once I got a load of Eddies Strat?, I tore my shit up trying to figure out what the hell was going on there.
Def Leppard’s “Photograph.”
“Turn it up”. 
I don’t know if any of this answers the OP, but I have always loved the solo (over dubbed? duet?) in Warren Zevon’s “Werewolves of London” at about 1:35 - to me it just oozes sexuality.
Then there was a hook from Santana’s Supernatural that is amazing, but I can’t put my finger on it right now.
Found it. Freaking hot video, to boot.
Roadhouse Blues by The Doors
Crazy On You and/or Barracuda by Heart
I’ll vote for sweet child of mine. I’ll also cast a vote for enter sandman, and scold everyone who mentioned eruption. Faster is not necessarily better, guys!
Is there a riff in Eruption? It’s just a guitar solo.
“Long Cool Woman” by The Hollies
Thunder Storm by Takanaka Masayoshi.
As yet unmentioned:
Opening riffs:
“Rock You Like a Hurricane”, Scorpions
“Bad to the Bone”, George Thorogood
“Life’s Been Good”, Joe Walsh
Walsh again on The Eagles’ “Life in the Fast Lane” and the James Gang’s “Walk Away”
“Black Betty”, Ram Jam
“Walk This Way”, Aerosmith
“School’s Out”, Alice Cooper (Glen Buxton on axe)
“Double Vision”, Foreigner
“Ain’t Talkin’ About Love”, Van Halen
“Cult of Personality”, Living Color
“The Ocean”, Led Zeppelin
“Don’t Fear the Reaper”, Blue Oyster Cult
“Man in the Box”, Alice in Chains
“More than a Feeling”, Boston
EVH on Micheal Jackson’s “Beat It”
AC/DC was namechecked upthread. Start with the openings of “You Shook Me All Night Long” and “Hells Bells”.
Other riffs:
The rhythm line of “Carry On Wayward Son” by Kansas
Sweet’s “Love is Like Oxygen”, after the synth intro
“La Grange”, ZZ Top
“Paperback Writer”, Beatles
Neal Schon’s closing riffs on Journey’s “Stone in Love”
Rush’s “Red Barchetta”
Ray Parker Jr on Boz Scagg’s “Jojo”
“Hold the Line”, Toto
That’s not a riff, it’s a solo. The term “riff” is shorthand for “refrain” - a repeating musical figure or phrase. In rock, that generally means it’s the rhythm guitar parts, not the leads.
I’ve got the Nuge on the brain today, so I have to add the intro to Great White Buffalo. That shit just rocks.
I really like Bowie’s opening riffs to “Ziggy Stardust” and “Rebel Rebel”.
Also, Pink Floyd’s “Shine On You Crazy Diamond” and “Run Like Hell”.
Hendrix’s “Are You Experienced”
Rolling Stones’ “Honky Tonk Woman” rhythm parts still blow me away, especially live. Great groove.