Nice thread!
But also a very hard question like jazzy said

So much has happened in my crazy life, and i am not easily defined by just one song, however, after thinking hard and long about it, there is one song that always, without exception brings me back to a certain point in time, which was possibly the happiest and most carefree of all my life so far.. Still hope i can top that period
To let you into my memories a bit:
I first heard this song when i was 17. It came to pass when i attended a very controversial school with possibly a bit too much freedom for most teenagers, and had made a pretty awkward "move" on the hottest Arts-teacher i had ever laid eyes on, she was pretty wild, looked like some rockin chick with that typical huge afro like head of curly blond hair, and was incredibly cool to talk with.
One day she came to collect us for class, when me and a mate were hanging out in the aula where we were playing a bit of ping-pong, after she told us to come, she turned her back to me and then my "move" was i actually fondled her bum
Don't know what got into me, but i couldn't resist and that is exactly what i blurted out when she looked back at me quite surprised..
Expecting a slap in the face, and some huge trouble afterwards with the principal, instead: she leaned towards me and whispered in my ear "we will have to discuss this after class"
You can probably imagine that i couldn't even act normal that whole class, and was actually shaking with anticipation as i really had a crush on her, but finally the class ended and we talked a bit, pretty awkwarsd still on my end, but she calmed me down a somewhat, and gave me her address where i was to come by later to have dinner, and then discuss the matter further
So i brought some weed, she cooked me one of my best meals ever, and vegetarian too, we drank the most amazing wine (never had drank wine before, after tasting it once when i was younger and spitting it back out, thinking all wine was disgusting like that)
Then we smoked some weed, all the while having great conversation about the magic of life, art people and whatnot.
Then she put on this song... i really spaced out, all the world was opening up to me, many things i had seen and heard off, things i had read and things i had pondered or imagined all fell into a place, where it made sense

It was like enlightenment, sort of. Then we kissed, and that lead to the most fantastic sexual experience i had so far, and the best morning after experience i have had in my whole life.
Now whenever i hear this song.. it's magic

I choose this song since it reflects both a turning point in my life, and one of my best memories ever.