Right now, I'm so down with 'Keep It Loose, Keep It Tight' by Amos Lee. I'm also happy kickin' it wid 'Food, Clothes, Medicine' by Aesop Rock.
If I've really got to push the boat out and pick an all-time favorite, I'd have to say that 'Ooh La La' by The Faces never ceases to bring a smile to my face...Oh, and 'Three Little Birds' by Bob Marley is perfect.