My opinion has shifted around a bit over the years. For a long time I would have said Ride the Lightning, and there are days where I still think that's the one I'd say. The Call of Ktulu is one of their finest songs, and there really isn't a weak track on the album (count me among those who enjoy Escape; sorry Mr. Hetfield!). There are also probably times when I would have said ...And Justice for All. I love the ambition on this one, and To Live is To Die and One are superb, but I think some of the other songs overstay their welcome a bit, and the production on this album really does have issues.
Ultimately, I think I have to go with The Black Album. The songwriting is just so well-honed on this one. It's one great song after another. It might not have a single track in my top five or even my top 10 (The Unforgiven would be close), but as a continuous listen I think it's the best one they have.
(Many of my favorite Metallica songs are on two albums that unfortunately aren't consistent enough to be among my favorites: Load and Reload. The strongest 80 minutes of music from those sessions would be my favorite Metallica album, but for better or for worse we have two mixed, mid-tier albums instead of one great one.)
Edit: I'm surprised that I'm the first vote for The Black Album. I know that among serious music fans the three albums before it are more popular choices, but I thought there were some enthusiastic TBA fans around here.