My favorites are
O Come, O Come, Emanuel
A mighty Fortress is our God
Were you there when they Crucified my Lord
Be still my Soul(slow mvt of Finlandia)
Let all Mortal Flesh Keep Silence
Be thou my Vision
There's also a nice one based off of the slow section of Jupiter from the Planets. I can't remember its title though.
[edit]-I forgot Dearest Jesus, what law have you broken, which is the hymn used in Bach's St. Matthew Passion