Slayer get a bit of shit because they weren't the most technically gifted band (although Dave Lombardo was/is a superb drummer).
I've gotta say you're way off to call Pantera a tuneless racket though. The heart and soul was always Dimebag, who was an absolutely phenomenal guitar player, and the current iteration of the band (without him and Vinnie) are a shadow. Phil's voice has always been about power rather than finesse, although his vocals on some of Down's stuff is surprisingly melodic.
Each to their own though, of course.
As for Sabbath, everyone's talking about the state of Ozzy when all I could think was how good Geezer Butler still is. Looks like he could smash out a world tour with no issues.