I'm a pretty big Metallica fan. I prefer the Ride the Lightning and Kill 'Em All albums, but Master of Puppets is a close contender.
As for the rest of their stuff. Most of Black Album, and everything after it was complete and utter fail. I just absolutely despise St. Anger and Death Magnetic for how much Metallica changed during those albums.
Megadeth and Slayer definitely still hold their title high as 2 of the big 4 of thrash. (What's Anthrax up to?) But Metallica has soiled their title imo.