As someone who has been considered this question several times in their life, I'm going to have to give multiple answers. Sorry about that. These aren't necessarily my favourite songs, just the ones that fit.
Maybe we should have a "Random song queries" thread, like "What's your favourite song about mutants" or "What's your favourite song to listen to in the rain" or something.
If we mean "The song that plays in your head whenever you feel a sense of direction e.g. going to the shops"...
If we mean "HOLY SHIT THERE IS NO FUCKING TOMORROW IF I JUST WIRE THIS SONG INTO MY BRAIN ON A LOOP AND IGNORE EVERYTHING"...
(This one probably seems a little out of place, it's complicated. Chiptune means a lot to me. Also you need big headphones, turned up really loud.)
If we mean a song that actually introduces you as a person...
If we mean "The song you're most prone to abruptly burst into at random intervals"...
If we mean "The song you're always ready to listen to i.e. a
literal soundtrack to your day-to-day life"...
If we mean "The 'Holy fuck I'm unstoppable' vacation video-montage song"...
If we mean "That song that just makes you drop everything and dance"...
If we mean "The song that describes how people perceive you"...
Ultimately, if I have to pick one song, which covers how I
generally feel
most of the time...
... I always feel a bit confused and hopeless.
There are plenty of my favourite songs that I would
like to be my theme tune, but I'm nowhere near cool enough.
On the other hand, there are also songs that I would like to be my theme tune if I wouldn't be judged so harshly... Fuck it. I live to make people happy. This song is me, dammit. An idealised me, anyway.
Phew! As bonuses for trawling through all that and loading a dozen videos, here's the song I want played at my wedding (if I ever have one)...
... and my funeral (if I ever have one, LOL).
Also,
I think I can guess what Pilot's will be.