Bands with the most creative riffing

I'd like to throw in a mention of Bolt Thrower for the most sucessful exploitation of a single riff. You know... the one that goes dun dun dun dun... every song in each of their albums has some variation of it.
 
Iron Maiden + Sabbath + Morbid Angel + Slayer bla bla bla
At The Gates for contributions to dual guitar tremolo polyphony
Gorguts is notable for innovations in rhythm and picking technique
Immortal - Is that a riff or a icestorm?
Thergothon - Totally different idea for use of the guitar
Iced Earth - THE GALLOP!!
 
Morbid Angel
Enslaved
Emperor (Particularly their later stuff)
Meshuggah
Opeth