I'd have to say Dyers Eve. I love the fast paced guitar and kick pedal, solo, and the song in general. I have many other favorites such as Fight Fire With Fire, Master of Puppets, and The Unforgiven.
I've been listening to Metallica for 25 years now. Orion's always been my favorite .. some of it has to do with the fact that it was their last album with Cliff and he does a bass solo in Orion so in a way it almost feels like a tribute song to him. All in all it's just a great instrumental.