Metallica. Far and away.
While I am at it...it's kind of funny what's happened to O:M in the years since its release, especially since Queensryche's fall from grace. It's commonly viewed by progressive metal fans as a seminal masterpiece, but when it was released, it actually took quite a while to get going commercially. Queensryche fans, who at the time were still working on the left turn the band took between The Warning and Rage For Order (another album that became a seminal classic years after the fact) were a bit befuddled by O:M upon release. My best friend was a huge Queensryche fan, but when I played O:M for him he didn't really like it. O:M was actually, in its time, a grower, something you listened to a lot before you got.
Master of Puppets? An instant, undeniable classic. YMMV of course, but it is kind of funny how distance has made people view some albums for old farts like me who were actually there.