Oh, this is a fun question!
Once--ages ago!-- I took either a magazine quiz or an online quiz on this topic and my result was "Walkin' on Sunshine".
In college, a few of my friends said they had a song that made them think of me every time... I was pretty disappointed to hear that it was Warehouse by Dave Matthews Band because I couldn't figure out why on earth the lyrics of the song would remind them of me. But then they explained that it wasn't the lyrics, but the music at around 3:10 & 4:15 in this video(and repeats a few more times after that) that they felt was quintessentially "me" --
https://www.youtube.com/watch?v=VksAuB0CJn4
Once they explained that, I loved it! I felt like it was even more special that they heard that music and said to each other, "doesn't this remind you of our friend?" than if they had chosen a more obvious lyrical connection like, say, "you have brown eyes, so this song called Brown Eyed Girl made us think of you, isn't that wild?!"
And even now when I've been feeling out of sorts for a while, I listen to those songs and remind myself that I'm actually a "walkin' on sunshine"/bright, bouncy musical interlude person, not an exhausted, complainy, blah person
But those 2 theme songs were assigned to me... I'll have to think about what I'd choose if I were to pick my own.