Well unfortunatley, I am old enough to have bough Kill 'Em All on record, yes I said it record, when it came out in the early 80's. So I remember exactly when they went 'over the hill' <cough>Jason Newstead</cough>. I think it will always be my favorite, nice and gritty, the levels are all off, it sounds like they recorded it in a garbage can. Pure long hair, sweaty, stinking of beer and whiskey, angst filled, genious.
Now that I'm older, I like the 'new stuff', anything from ...And justice for all, and on. There are some good bands cranking out the tasty licks out there now, but bands like Motorhead and Metallica are all of their granddaddies.