I'll get this one started with Led Zeppelin's "No Quarter"

On Bass It's just about anything from hooky from New Order.

the riff in "im looking through you"...amazing sound!

On I always start air guitaring when I hear it, it's the starting riffs on Metallica's "For whom the bells toll"
There are four riffs catchy as hell before the vocals even start.

Probably the keyboard in "96 Tears"

Devil's Dance by Metallica. Very catchy riffs.

Heard "Rebel Rebel" the other day in a music store. That's gotta be on the short list.

Recently, The Dead 60s - "Riot on the Radio"

"Like Father Like Son" by Exodus, the riff after the lyric "The pattern of hurting began generations before."

The 3 "A" chords which open AC/DC's "Highway To Hell".

"Limelight"-Rush

Probably the main riff in Ween's "Roses are Free."

Symtom Of The Universe from Sabbath \m/

I've always been partial to "Andy Warhol" by David Bowie.

Anything from this album:

Do the Dew by a Certain Ratio

or is it Du the Dew, whatever.

Another vote for AC/DC - my candidate is "Whole Lotta Rosie".

There is only one riff, A-Bb-B-E, A-Bb-B-E-E

I immediately thought of Kashmir.

Man on the Silver Mountain

The beginning of "Sweet Child Of Mine."

For me it's definitely a Beatles riff but I can't pick just one.

Day Tripper, I Feel Fine, Revolution, And Your Bird Can Sing, Don't Let Me Down, Ticket To Ride, etc etc

the industrial sounding noise on Techno Animal's Cruise Mode 101 ft. Rubberroom.

I can't limit myself to one, so I'll mention three.

AC/DC - Dirty Deeds Done Dirt Cheap
My love for this song, and especially the riff, is such that I now only listen to it during special occasions. It marks the beginning of every holiday.

Muddy Waters - Mannish Boy
This one's especially great because you can hum it with DIY lyrics about what you're doing at the moment (a la Samuel L. Jackson's character in "The Long Kiss Goodnight").

Chuck Berry - Johnny B. Goode
Irresistible and timeless.