For me, I'd rank 4 of their first 5 albums as quintessential metal albums.
Binge and Purge is outstanding, one of the best live albums of all time (metal or otherwise)
Load and REload were a change in direction but are both solid.
Symphony is pretty cool with a metal band mixing it up with an orchestra.
Garage Days has some good covers and some weaker covers.
St Anger is garbage. 1.5 good songs and that's about it. Worst drum sound in the history of metal. Lulu is the most embarrassing thing Metallica has ever done and that says a lot considering they put out St Anger. Death Magnetic is decent. Not good, not great, not bad either.