Have an angel walk with her by Martyn Joseph. I don't even understand what it's about entirely, and Martyn AIUI is fairly cagey about it (it's one of those songs that's taken on a life of its own), but for me it'll always remind me of one of the first times I spend with my ex - we'd just got together on a week-long mission thing, and after it was all over, we spent the Saturday afternoon mooching around Manchester together, during which I picked up a tape of Full Color, Black and White (the album featuring the song in question). Come the early evening, I had to return to London. After the emotional farewell, I settled down to listen to the tape. That song just really hit me. I guess it just seemed right for emotional partings.
The Survivors by Pet Shop Boys comes from a similar time in my life and had a similar effect on me first time I heard it. Less so now, but it's still an amazingly moving song, IMO.