I'll narrow it down to three because it's hard to pick one..

Do For Love - 2Pac - It's funny, this may be one of the only songs from 2Pac that could be deemed as a love song. But it really works, with a mellow beat and honest lyrics, it's one of my favourite's from the man.

All I Have to Give - Backstreet Boys - This song is just a great relationship ballad.

Two Faces - Bruce Springsteen - A typical Bruce song about a not-so perfect couple, but it's great in the way it potray's itself through the music & lyric's.