At the moment it has to be this one:
[youtube]http://www.youtube.com/watch?v=AB6sOhQan9Y[/youtube]
mostly due to the fact that the music is so simplistic and peaceful when the boss battle it is tied to is very tense.
Also it is a great counter to almost every other soundtrack for dark souls where most of the others are very bombastic orchestral pieces with lots of chanting in the background.
an example:
[youtube]http://www.youtube.com/watch?v=6ukC3N1rkzQ[/youtube]
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I am so omniscient, if there were to be two omniscience's I would be both! Prepare yourselves for the subjugation! - Ziltoid The Omniscient.