Check out Billy Gould from Faith No More. I think he's one of the greats. What makes him particularly great to me is his judgement for when to keep it really simple but absolutely solid. His use of slap to accent parts and never get carried away with it is excellent. His pick tone is superb as well. Check out Midlife Crisis and From Out Of Nowhere
Tim Commerford of Rage's ability to provide a funky groove behind rather chaotic guitar and vocals is excellent, he just brings the band together. Check out Renegades of Funk and Bulls on Parade
Geezer Butler of Black Sabbath is just genius. His parts are often more complex than the guitar part. Check our NIB and Iron Man
Cliff Burton of Metallica (up to Master of Puppets) was the master of making the bass the lead instrument. Check out Orion and Anesthesia (pulling teeth)