I agree with Fortress that there are a lot of metal bands that are fantastic, far superior to many of the ones named here. Metallica and Pantera are probably my two favorites all around of all time, but bands like Children of Bodom and Lamb of God are hard to beat. Bodom is just good hard metal, and LOG is just the epitome of southern metal thrash...kind of Pantera to the extreme.
I have to say, bands like Iron Maiden, Def Lep, Ozzy and Priest, these bands don't even begin to enter the same category as the four bands I just mentioned above. Sure, taste dictates a lot and everyone's is different, but the Maiden's. Ozzy, etc., none of these could be put into the same metal category as the above four.
Metallica
Pantera
Children of Bodom
Lamb of God
Sepultura
Divine Heresy
Agony Scene
Omnium Gatherum
Arch Enemy
In Flames
Scar Symmetry
That's a metal playlist of bands...real, true metal.