My favorite instrumental by them would have to be Orion, cause of the Bass solo, but 'To live is to die' would be a very close second.
call of ktulu, is good too.......... but, I do agree about ORION. the best metallica instrumental, maybe the best heavy metal instrumental.
The Call, easily. The whole sudden and massive increase of complexity and variety from Kill 'em all to Ride the lightning is amazing, and this song shows that so incredibly well. The way the the tension is increased more and more towards the climax is simply delicious. I love this song.