Iommi, along with Jimmy Page, is the master of the heavy riff. I also have a lot of respect for him because he plays with prosthetic fingertips on his fretting hand.
Ritchie Blackmore, when he was focused and on his game was a master too. Not the over played Smoke on the Water.
Stargazer from “Rainbow Rising”
Pictures of Home from “Machinehead”
Heavy riffs and blinding solos
Not necessarily the best, but in the top tier and my favorite