Master of Puppets both for tone/songs and for being the one that literally knocked me on my ass in awe when I first heard it. So thick and tasty.
At the time, there really wasn't a big four in my mind. There was no delineation between the "big" bands and the little bands. They were all equals to us in the middle of fuck-off nowhere midwest.
So, Master of Puppets, Among the Living, The New Order (Testament), The Years of Decay (Overkill), Fabulous Disaster (Exodus), Reign in Blood, Peace Sells, Handle with Care (Nuclear Assault) all got regular rotation through that period of time. Some bands had better albums later, but those were the ones that stick in my craw due to the lovely mixture of nostalgia, time to listen to them over and over and over and over and over, and being new enough to the genre to not be utterly burnt out on it. Pretty much all of them could be sung, some of them even played, all the way through note for note from memory. To me, that's a sign of some form of greatness, even if it's only in my puny brain pan.
That was also the first one I got from any of the bands. I don't own any Metallica, but I'm not really interested. I much prefer the other 3 of the big 4