Of course Metallica beats Megadeth hands down all the way!! Again, what happened to them after the Black Album? I can't believe the transformation!! They better give it up and retire, rather than releasing such crap just for the money.
If you really want to hear some great metal, especially Power and Melodic Death, I strongly suggest you start to have a look at the other side of the Atlantic. Great bands out there, much better than the "Nu metal" crap you often hear in the States. However, there are several US bands that I still enjoy and discovered not a lond time ago, such as Slough Feg, Twisted Tower Dire, Division (the last two from Virginia).