It was the acoustic/strings only version of "Eleanor Rigby" that I heard at age 21 that made me fall in love with the cello. It still makes my spine tingle for some reason. There's also a few others. One of them is Craig Armstrong's "Laura's Theme." I can't think of any other song that is more melancholy than that one. The Swan by Saint Saens. I like some of Yo-Yo Ma. The more traditional and Baroque stuff.