Yeah - Master of Puppets. Recommending this album to a dedicated metal fan in 2008 is like "recommending" Pixy Stix to a five-year-old; they already know what it is, and you can guarantee they've already found it palatable.
Nevertheless, it's a testament to Metallica's glory years that they released their best album a month before I was born, and it's still one of the heaviest, most intense and dedicated metal albums of the recording age (though some of Baron von Armageddon's early wax cylinder pressings from the turn of the century shredded as only turn-of-the-century riffage could).
This album's razor-sharp metal aesthetic gives it most of its appeal. It's neither as brutal or sadistic as Slayer's contemporary offerings, nor as fantastical, overblown and ridiculous as. . . well, as pretty much every other metal album of the time. It's much crisper than Metallica's early releases, but showcases the band at the peak of their talents, before they beefed up the production and lost the sense of adventure they showed on their first three albums.
"Battery" may very well be the very best Metallica opener of all time. It's pure fast-as-nails punk and it's unforgettable. The album runs through a good mix of fast and slow tracks, all of them very technical and all of them heavy. The first half a dozen times you lie awake at night with "Master! MASTER!" echoing through your head, you might even enjoy it.