Wow this is like forcing me to pick my favorite child. I will have to go with AJFA since that was the album that introduced them to me. I've been hooked ever since. I even love St.Anger to an extent (the drums and vocals are the weakest of all the albums), but some of the riffs really kick *** on it. Death Magnetic (minus the clipping) is really awesome too. In fact I'd say it could have been the perfect album to come out right after Justice.
when I was 16, it was Kill 'Em All. As I approach 40, I've somehow come back to KEA. I think I've just lost the patience for the longer MOP and AJFA songs. And, like most people, I grew to despise the group for all of their post-AJFA releases.