1. 90's- Tons of variety, even in the mainstream if you looked hard enough. Something fresh and exciting was always coming out from tons of different bands.
2. 70's- Purity in music. This would be #1 if the 70's didn't have such a limited selection. I imagine it took much more patience to be a music lover in the 70's, just because you would have to wait so long for one of your favorite bands to release something. Today you can just occupy yourself with some other crap until what you're waiting for comes out.
3. 00's- So far it's been weak. Mainstream metal sucks once again, and I'm sorry, but most underground metal isn't much better either. It's all been dumbed down and sapped of all originality.
4. 80's- Oh god, I don't even want to start. But I will. The 80's SUCKED. It took all of the coolness out of being in a metal band or even being a metalhead. That word damn near makes me puke, and that's because of the 80's. Even the bands I'm supposed to like (Old Metallica and Judas Priest, to name a couple) have no impact on me. And Glam/Hair metal, whatever you want to call it, made rock a joke for a long time.
*ahem*
BUT Maiden is the greatest band of all time, hands down, 80's or not.