The list as usual, sucks.
Some great riffs on extreme metal IMO:
Ulver - I Troldskog Faren Vild (both main riffs are awesome), Soelen Gaaer Bag Aase Need (clean vocals riff).
Hypocrisy - Osculum Obscenum - intro and chorus riff mainly (but the entire song is a kick in ur ass).
Burzum - Spell Of Destruction - verse.
Emperor - I am the black wizards , Inno a Satana (intro) and The Wanderer.
Morbid Angel - Where the slime live (not just the guitar, also the drum pattern is now a MA trademark, at least Bloodbath's Mock the cross, Opeth's Master's Apprentices and Luciferion's New World To See reminds me that song).
Satyricon - Hvite Krist Dod - Mother North
Immortal - Blashyrkh (since the clean riff till vocals begins).
Death has A LOT of amazing riffs... some of my favs:
Scavenger Of Human Sorrow 1st riff
Symbolic 1º and chorus riffs.
Crystal Mountain 1º riff
Lack Of Comprehension pre-chorus and chorus riffs.
Together As One intro
Pull The Plug pre chorus riff.