You guys put System Of A Down, Marylin Manson and Nine Inch Nails in a metal play list? 
-rgdot
Well, System Of A Down is nu-metal, so it sorta qualifies... and if you put oldsk00l snobbery aside, they've actually made a couple of OK tracks. I wouldn't call anything Manson has made
metal (portrait of an american family is more punkish than it's metal), but imho a lot of his tracks fit decently in a metal playlist (if you're not going for purist

). As for NIN, well, Reznor has varied his style quite a bit across his albums. Broken fits
perfectly in an aggressive metal playlist.
For some brutal stuff, check out
DIE and
Dawn Of Demise - those are guaranteed
\m/etal, with a vengeance and all. Disclaimer: I know some of the guys from those two bands, so you could see this as a blatant ad

Oh, and while it doesn't really belong in an "ultimate metal playlist" (we've already drifted a bit OT), you might want to check out
volbeat. It's in the softer end, more rock than metal really... although I suppose you could invent a genre called Elvis-metal for them
