Greatest Single Song of All Time

I know it's basically just a matter of opinion, but I'd like to know what you think. I'm just curious. For me, that's a VERY hard question. Music for me is completely based on my current mood. I listen to everything from The Beatles, to Tupac, to Gogol Bordello, to The Black Dahlia Murder. I even dig a lot of 80's and 90's country. I just like music. I'm racking my brain trying to figure out the single song I would consider the greatest of all time. One that I seek out, no matter what my mood is. I honestly can't come up with a single one. I considered Stairway to Heaven, because it truly is a masterpiece, but There are too many other songs in other genres that could also apply. Maybe you'll have better luck coming up with an answer.