I love James Horner's haunting soundtrack, so much so that my husband asked if I ever listen to anything else. LOL! (Yes, I do.)
My favorites are "Take Her to Sea, Mr. Murdock," which evokes the sweeping beauty of the visual sequence. (My favorite part of the film, btw.) I also like the tracks with Rose's theme (sorry, Susan), especially "Unable to Stay, Unwilling to Leave" because it is heart-wrenchingly sorrowful and so appropriate for the disaster. It makes me think of all the families lost loved ones.
As others have mentioned, "A Building Panic" and "Death of Titanic" are also haunting, but I like "An Ocean of Memories" because it ends on an optimistic tone. To me, that evokes the need to go on with life after a crisis. I know that's not a subtle theme in Cameron's film, but it's something we all have to do sooner or later.
Best Wishes,
Cathy