Pretty tough decision.
It comes down to the first 4 albums.
Kill em All is great but they just topped that one multiple times.
I like Master of Puppets as much as the next person, but Leper Messiah was never a song I liked, and it always brought the album down for me.
Ride The Lightning was my favorite for a long time, but I don't need Escape in my life ever again.
Justice obviously has bass issues, but every song is just one all time slugger after another. It's their angriest album, (but not angst driven) heaviest, lyrically one of their best, and for the first time, Lars sounds like a competent drummer.
Justice for me, but only by a hair. RTL and MoP are more legendary, but each has one of those weak tracks that Justice doesn't have IMO.
Black Album is good, but half the album is radio hits that have been WAY overplayed in the last 30 years. The other stuff is solid, and once in a blue moon, I enjoy a sit through of this album, if only for the surprises of "oh I forgot about this part/riff" during the non-singles tunes.
Since then I like some stuff but I cherry pick at this point.
AJFA
RTL
MoP
KEA
s/t
HTSD
DM
L/R
any Metallica interview
St. Anger