If I had to pick an overall favorite song, I'd go with Kid Cudi - Cudi Zone
I don't listen to it much anymore, but it gives me nostalgia, and is special to me.
At the moment my favourite song is still Tove Lo - Stay High (Habits Remix) ft. Hippie Sabotage. Has been since it's come out. As for all time, I wouldn't have a clue.