Memorable riffs

Satyricon - Forhekset (opening riff) my favorite riff of all time probably
Soilwork - Millionflame (chorus and verse riffs)
Decapitated - Human's Dust (opening riff)
Aeternus - Burning The Shroud (opening riff)
Arcane Sun - Longing For Eden's Rain (the whole song)
Darkane - July 1999 (opening riff)
Nevermore - Dead Heart, In A Dead World (verse riff)

I probably forgot a bunch of great riffs.
 
Suffocation - Torn Into Enthrallment (after the intro)
Immolation - Close to a World Below (the slow opening and finishing riff)
Behemoth - As Above So Below (main riff)
Decapitated - Spheres of Madness (main riff, not the intro)
Cryptopsy - Graves of the Fathers (both slow riffs)

I could name a thousand......
 
The riffs at 1:24 and 2:14 in Cauterization by Dark Angel
Opening riff in Exacting My Revenge by Diecast
Main riff in 0:23 in Idol In Human Form by Beyond the Sixth Seal
Main riff in Dracula With Glasses by Burnt By The Sun
Opening riff in Nailed to the Cross by Destruction
Main riff in My Last Serenade by Killswitch Engage
Opening riff in To the Sons of Man by Killswitch Engage
The riffs 0:46 in Eternal Gates of Hell by Dead To Fall (Sad that the vocals sucks ass)
The riff 0:27 in Condemned to Suffer by All Out War
Main riff + firstsolo in A.I.R by Anthrax
Opening riff in Gung-Ho by Anthrax
Main riffs (the ones in the chorus sucks though) in Son of a Gun, Daughter of a Slaughter by Terror 2000
Main riffs in the following Defleshed songs: Entering My Yesterdays, Brakefailure, Thorns of a Black Rose (listen when the brake comes, fucking awesome), Under the Blade etc

I don´t wanna play anymore, Mr Gein!!!
 
Lets see..

Black Sabbath-The Wizard The first one (I think anyways)
Children of Bodom-the Lake Bodom intro lead/riff thing
Death-The beginning of the Philosopher
Deep Purple-Smoke on the Water (whoever hates this riff, no matter how much it is played, should die, now)
Dream Theater-The one after the vocal solo in Pull Me Under
Dream Theater-The first guitar riff of Home
In Flames-Main riff of Jotun
Led Zeppelin-The first/main one of The Ocean
Megadeth-All of Wake Up Dead
Megadeth-The beginning of Tornado of Souls
Metallica-Seek and Destroy (don't bitch about how much you've heard it, it is great)
Metallica-The second heavy riff of Orion
Metallica-EVERYTHING in the song ...And Justice for All
Nirvana-All Apologies
Opeth-The intro to Advent
Opeth-The intro to Nectar
Opeth-the heavy riff of To Bid You Farewell
Opeth-the acoustic/distorted buildup of Demon of hte Fall
Opeth-The first heavy riff of The Funeral Portrait
Opeth-The last heavy riff of Blackwater Park
Opeth-the intro to Windowpane
Ozzy-Crazy Train (its just classic, everyone knows it)
Pantera-Floods
Porcupine Tree-the acoustic part in Strip the Soul
Shadows Fall-the intro of Of One Blood
Slayer-Raining Blood
Symphony X-the second one in Smoke and Mirrors


thats enough...