Hetfield was basically monotone throughout much of the '80s for the heavier stuff, and tended to sing sharp, but from TBA onwards, I've really liked his vocals. Load/Reload are probably his best vocals, and he has a great tone.
I like Mustaine's vocals too, and he had a greater vocal range on the '80s stuff, but he's an acquired taste, and his best isn't as good as Hetfield's best imo.
Hetfield it is.