Not in order.
Opeth (old stuff, new albums meh)
Burnt by the sun
and Pig destroyer just to name a few.
Their latest album "Distance Over Time" is a masterpiece! I easily have it in my top 5 DT albums of all time. Funny too because it followed "The Astonishing" which I consider one of their worst albums of all time.
Some good lists in here. I'd add Sepultura, for sure... Heaviest show I ever attended, and they were incredible in their prime. Also, RATM would probably considered their own brand of heavy.
Newer stuff: FFDP, Bullet for my Valentine, Halestorm, In this Moment.
It's hard to pick a single favorite, as that largely would depend on my mood. Metallica would probably be my choice, although I think Pantera has a strong case with its three album run of Cowboys, Vulgar Display, and Far Beyond Driven.