I could never pick just one... I've got so many for different moods!
My favourite "old" song is Lola by The Kinks, and one of my current favourite "could listen to this all day" songs is Tickle Me Pink by Johnny Flynn. And basically anything by Alkaline Trio, particularly when Dan Andriano is singing, because I love the lyrics - although not so much when I'm driving my mother and singing along in the car and forget to censor the swearing...