In my mind, the definitive slide guitar recording, at least from the blues/gospel tradition, is "Jesus Make up my Dying Bed" by the Reverend Blind Willie Johnson. The control he has over his instrument - it blows me away. Great synchopation also. A word of caution, though - his voice is not for the faint of heart.

I think I've seen it on a compilation CD but I "obtained" it from the radio - a show by a guy who collected 78's.