This is literally an impossible question to answer.
If you held a gun to my head and said, "Sarah, pick one favorite song," I'd be like, "Please stop holding a gun to my head," and when you realized that it was stupid to be threatening my life over a song, you would lower the gun and contemplate what you were doing, which would give me the opportunity to Judo Chop your wrist to get the gun out of your hand while holding you in a Full Nelson until the cops got there. Incidentally, it wouldn't take them long to get there, because they were watching from across the street and one of them says, "Holy Jeez, that person has a gun to that girl's head." This is also how they knew to come over. Because they saw it.
Point being, all this would transpire because I wouldn't know what to tell you. I can tell you favorite artists, I can tell you songs I love, but I can't actually give you one song that is My Song. So I'll give you a few from which to choose and the circumstances under which I love them. I hope that's OK, but I'm guessing it won't be, since you were the one holding a gun to my head a few minutes ago.
Best Song That Reminds Me of Middle School But That I Still Sing Along To
-- *NSync's "I Want You Back"
This song was 'DA BOMB' in middle school. (Do people still say that? Did anyone ever say that?) It's catchy, it's poppy, it's about a boy wanting a girl back because he did her wrong (which. . .I had no experience with, but I took their word for it that that's the way it goes.) I love this song to this day and will still unabashedly play it with the windows rolled down whilst rocking out. As much as one can rock out to a boy band.
Best Song to Sing During Karaoke If I Were Willing to Sing in Public
--Carrie Underwood "Before He Cheats"
I don't sing. No, wait, that's a lie. I don't sing well. Therefore, I do not sing in public. However, there are times when you're out at a bar (I would imagine) that your friends are like, "Come on! Sing! Get up and do karaoke!" And if that were to ever happen to me, this is the song I'd sing. I don't know why, but I've always imagined getting up and getting all into it and tossing my hair in a sexy sort of way that says "Yeah, you're not going to cheat on ME!"
. . .I don't know.
Best Song That Will Immediately Put You in the Mood for Lovin'
-- Maroon 5 "Kiwi"
This song is dirty. There's no other way to put it except that it's dirty as hell. And I love it. (This isn't an actual music video, as Maroon 5 never recorded one for this song. It's just the song.)
So there's three. I have a list of like 6 more, but I started this post like 12 hours ago, and I feel like it's about time to move on. Maybe I'll revisit the idea of Music for Special Occasions later.