This one is pretty easy for me.
When we got married, I DJ'd our own wedding with a series of playlists for each part of the night. Then we gave out a mixtape that represented tracks from the night.
From there, every year on our anniversary, we make another playlist. We've been married 15 years, so that's 16 mixtapes of music. The whole thing is right here, all 16 - https://open.spotify.com/playlist/5sXokGoRzMdrT7CykXmfx2?si=f8c05f8ea7004f3d
If I had to pick 1 song, it would be the lead off song, Decemberists "The Crane Wife 1 & 2". https://songwhip.com/the-decemberists/the-crane-wife-1-and-2
The Crane Wife story and then album were part of our relationship while we were dating, then the night the Decemberists came to town touring the Crane Wife, that was the night I proposed, then we went to go see them. We still have the concert posters from that night up in our bedroom.
So yeah.