I can't begin to narrow it down to one song, but the one that pops into my head that has had steady rotation for many years is 3x5 by John Mayer, though anything by Jimmy Buffett (other than Elvis Imitators and Steamer) is always welcome at any moment in my life.
Edit: Looking at my iTunes, the song with the most play is Happy Ever After (Now and Then) by Jimmy Buffett, closely followed by Trying to Reason with Hurricane Season, Jamaica Mistaica and U2's Sometimes You Can't Make It On Your Own. I haven't updated the iPod in a while, so those numbers may have changed.
Edited edit: 17 of the top 20 songs are by Jimmy Buffett. U2 (once) and Albannach (twice) snuck into that group as well.
Edited edited edit: Here is a list of my top artists and songs for the past couple three years
I'm sure it's not completely accurate because I don't upload the playlist from my iPod, it doesn't read my XM radio, and has no idea what I listen to on my cars' mp3 players, but you can see who my favoritest is most of the time when I'm at home...