1. Iron Maiden
Never done a bad song. Yes, there are some that aren't as good as others, but they aren't bad, they're just less excellent. I rate them at the top because there isn't a song that I would skip if it came up on random.
2. Edguy
Because power metal. There's just something about it that's simultaneously hilarious, epic and damned funky listening.
3. Rammstein
Heavy, chugging, awesome riffs. I barely speak any German (hopelessly monolingual, although I can order beer in nearly two dozen languages, which is really all you need), but considering some of their lyrics, this might not actually be a bad thing.
4. Opeth
Incredibly well put together, musically. Vocals are a bit shouty and incoherent for me, but the instrumentals make up for it.
5. Metallica
Oh come on, you weren't expecting me to get through this list without including Metallica, were you? When they're good, they're f*cking awesome, when they're not, they're still good. I do have to be in the right mood for Metallica though, to be honest.
Honourable mentions:
Judas Priest - for being Iron Maiden on helium and adrenaline.
Steel Panther - for being a hilarious parody of hair metal at its greatest.
Pantera - for doing a half dozen absolutely brilliant songs.
Black Sabbath - for being Black Sabbath.
Die Apocalyptischen Reiter - for doing a f*ckawesome cover of Ghostriders in the Sky.
Dethklok - for having the best debut album ever.
Bathory - for basically inventing black metal and all its subgenres.