I am not even that big a metal fan (and usually do not like thrash), but Master of Puppets is pretty much one awesome song after another.
I remember when I got into it, being shocked at how melodic some of the songs were, considering how heavy and fast they were. I think that is why it is so popular: it has a raw energy and power that is infectious, and also has memorable melodies, so you get the best of both worlds.
Nothing else they've done comes close to it, IMO.