I actually made up my own 2-disc 'hits' compilation to listen to in the car a few years ago so I didn't have carry all their cds, and I made something like this which I think are their biggest songs/hits. This was prior to St Anger being released, so there are no songs from there on it, but it fit nicely onto 2 almost-full cds.

Hit the Lights
Jump In The Fire
Whiplash
For Whom The Bell Tolls
Fade to Black
Creeping Death
Battery
Master of Puppets
Welcome Home (Sanitarium)
Last Caress/Green Hell
Blackened
One
Harvester of Sorrow

Cd 2
Enter Sandman
Sad but True
The Unforgiven
Wherever I may Roam
Nothing Else Matters
Until It Sleeps
King Nothing
Hero of the Day
Mama Said
Fuel
The Memory Remains
The Unforgiven II
Turn the Page
Whiskey in the Jar
Die, Die My Darling