I don't know what song is the saddest but I'm going to go out on a limb and bet it's a Hank Williams tune.
If your asking me, it's Hank Williams - I'm So Lonesome I Could Cry.
A close second might be Keith Whitley's - I Wonder, Do You Think Of Me. This tune was released in August of 89, three months after Whitley killed himself. The Song is about Lori Morgan, who left him and would eventually hook up with Troy Aikman.
It's interesting, the similarities between Hank Williams and Keith Whitley. Both stupid talented and both extremely tortured artists. Damn shame, both of them.
The one song that I always listen to and never fails to make me think is a tune from Craig Morgan called I was Almost Home. Makes me think of every vet that stateside never treated well, and there are a lot of them unfortunately.
Lastly, and this one is a personal one for me, Zach Brown Band's - Highway 20 Drive. It reminds me of myself and my oldest Son, ABQJr. when he was young and his Mom and I separated. He was really young and we were really dumb and I'll just say that there is no pain, IMO, quit like having to leave your child, every week. It's like cutting out your own heart out over and over every Sunday.