Not as simple as it seems...post the best song you think has ever been written. This does NOT mean it has to be your favorite. I hate the Rolling Stones, but I think Start Me Up is absolute genius. The song must transcend it's era both musically and lyrically as well as give goosebumps to everyone within listening distance everytime it's heard.
My top four:
Start Me UP
Happy Birthday (Don't laugh, meets all the criteria)
Shubert's Ave Maria, as sung by Pavarotti
And finally, by the only band that really mattered:
Rudie Can't Fail
YouTube - Rudie Can't Fail - The Clash