What is Your Favorite Metallica Album? [POLL]

A coworker and I were talking about music today when the conversation shifted over to Metallica's best album,  which is always debatable.  He likes 'Kill 'Em All' while I think 'Master of Puppets' was the best.  So I thought I would take this little debate to you. What is your favorite Metallica album? Don't get me wrong, I love the hell out of 'Kill 'Em All' but I don't think that album, or any other album, can hold a candle to 'Master of Puppets'.  It was groundbreaking.  It was melodic.  It was heavy.  It was bad ass! It was also one of the albums that changed my life.

In my opinion, with exception to 'Load', 'Reload', and 'St. Anger (OK, basically everything after the 'Black Album), all of Metallica's albums have been bad ass, but 'Master of Puppets', by far, is my favorite.  What is yours?