"Kill 'Em All" is my favorite because it's the most raw and aggressive sounding. Also, I tend to be one of those "the first album is the best" type of guys so that's probably a contributing factor.

RtL and MoP are also great albums too. I don't like the guitar sound on MoP though, it's too lifeless and sterile. That's when they switched over to Mesa gear though, instead of Marshall which they used on their first album. Bad move, Marshall rules the metal world.

I can't stand any other Metallica albums beyond the first three.