Black Sabbath and Blue Cheer were co-owners of the idea of "riff" in the first place, until Kyuss picked it up after they dropped it. Kyuss owns it to this day, and Queens of the Stone Age rents it out constantly.
Really, though, the songs that stand out the most for me in terms of their riff are:
Mondo Generator - Like You Want
Parchman Farm - Say Yeah
The Bevis Frond - Maybe
Masters of Reality - Kill The King
Kyuss - any song at all
Dozer - Man Made Mountain, The Hills Have Eyes
500 Ft. of Pipe - Marshmallow
Buffalo - The Prophet
Desert Sessions 1+2 - Screamin' Eagle
Dead meadow - Drifting Down Streams, Good Moanin', The White Worm
Blue Cheer - Parchman Farm, Babylon
ZZ Top - Precious and Grace
Black Sabbath - Into The Void, N.I.B.
The Black Keys - Thickfreakness, If You See Me, Till I Get My Way
Baby Woodrose - Pouring Water
Queens of the Stone Age - If Only, Walkin' On The Sidewalks, Mexicola, God is On the Radio, 18 AD
Hope this helps.