Anything that Tony Iommi wrote!
Also alot of Opeths' songs, I can't think of any of my head though.
Deicide - Lunitic of God's Creation and Trifixion.
Just to name a few.
S.O.D. - March of the S.O.D. (I maintain that this is still one of the heaviest riffs ever done. Still holds up 20+ years later)
Soundgarden - Outshined (I know...not metal but still heavy as fuck)
Slayer - Raining Blood
Iron Maiden - Sun And Steel
Uncle Slam - Weirdo Man