Tomaso Giovanni Albinoni - Adagio in G minor (Berlin Philarmonic / Karajan - for strings and organ c-moll)
(You can find more information on the composer and piece here.)
Clint Mansell (featuring Kronos Quartet) - Winter: Lux Æterna
Both tracks are purely instrumental... but through melody alone just make my heart sink... they just drain the happiness out of me. I don't think I really need to go into more detail than that... but while they are some of my favorite pieces of music (Albinoni's Adagio being by far one of the foremost)... I don't dare listen to them on repeat very often.
(I'm not sure how long those tracks will be available, I just googled for them and linked what I found. The first is not the actual version I prefer to listen to, but should give you an idea none the less.)