Manowar. These guys take "Viking Heavy Metal" to a new level. They only sing about battle and killing. Cool, eh?
Metallica. People who say "Metallica are too mainstream for me" are really saying "I'm too much of pussy to handle Metallica, I'm going to go listen to N*Synk." Metallica are heavy metal. What is wrong about that? Nothing. Listen to Metallica.
Iron Maiden. British people who sing about the apocalypse. Nothing much more to it, except that they kick ass. More ass than you anyway.
Rammstein. Hell yeah! German heavy metal. Ever want to learn how to speak German? Listen to Rammstein. You may not know what the hell they are saying, but if you repeat it while using an angry voice, you'll get the point across.
Pantera. Badasses at heart. They are the Cowboys from Hell, if that makes any difference to you.