As the title says....which song still haunts you emotionally? And why? A (freely accessible) link might greatly bring your point across.
Mine would be: (Typhoon - "Empiricist") https://www.youtube.com/watch?v=9E328pIZWFM
The lyrics go deep and it just touches me. No otherwise special attachment.
https://youtu.be/k27gPXCI7AY
This song plays at one of the hardest moments of the visual novel Steins;Gate. It also starts the same moment in the anime, but the VN truly nails the nature of the moment better.
Without spoiling it, all I can say is that the scene is just as sad as the song is.
Edit: The S;G0 version has strings and captures even more of the feelings, although the original is more than enough on its own.
https://youtu.be/c15uUuo3acg