Definitely Justice for me, that album is a masterpiece of progressive thrash. The title track, Blackened, Dyer's Eve, Frayed Ends of Sanity, One; hell yeah. The famous terrible production aside (there's literally no discernable bass due to the alleged inner band issues at the time), it's some good stuff.
After that, I'd say Master of Puppets of course then my third is Hardwired, love that album.
And I will say this; St. Anger has some good songs on it, and wouldn't have been as widely dunked on if any other band then Metallica put it out. With a bit of significant trimming on every song, and re-recording the drums, I think it could actually be good. Unnamed Feeling, the title track, Frantic; there's some good ideas in there.