I won't even pretend to be able to pick a song. I might have to go for ...And Justice for favorite complete album, though, even though a bunch of songs I really like are on other ones. For some odd reason, I've never really like Kill 'Em All, despite owning all the early (read: good) albums on tape since...well, since back when music still came on tapes. Heh. Always liked the rest of their stuff from the 80s, though, and I'm one of those weird people who still mostly liked them all the way through Reload, even if they weren't as good anymore at that point.
St. Anger was so bad that I totally gave up hope on them ever doing anything worth my time again, and after hearing how atrociously mixed/mastered the first single from Death Magnetic was, I never bothered listening to the album (although I did kind of want to try grabbing the version from GH, which is supposed to sound massively better, but I kind of forgot about it). The clipping is so bad that it's almost unlistenable for me. I thought it must've just been a badly-encoded MP3 my friend sent me (LARS SMASH), but then the official release sounded like ass, too...
Oh. Can't forget to mention S&M as one of the albums I particularly like, along with the earlier stuff. I'm a sucker for weird, hybrid forms of music like that (and well-done mashups of things you wouldn't think would work together, and bizarre cross-genre covers of songs that shouldn't sound good but do). I guess that's probably why all the wacky metal coming out of Europe over the past several years makes me giddy like a schoolgirl, like Epica (and maybe stuff like Finntroll, but I haven't gotten around to listening to enough yet to make up my mind).