Too many to choose from!!! Here's some of my favorites:
Let It Be - The Beatles
Crossroads - Cream
Little Wing - Jimi Hendrix
Imagine - John Lennon
And So it Goes - Billy Joel
Hey Hey What Can I Do - Led Zeppelin
You Can't Do That - The Beatles (and really anything else by the Beatles)
Wild Horses - The Rolling Stones
Take It Easy - The Eagles
Just My Imagination - The Temptations
Canon in D - Pachelbel
Sonata Pathétique - Beethoven
Moonlight Sonata - Beethoven
Jesu, Joy of Man's Desiring - J.S. Bach (Leo Kottke's classical guitar rendition of this piece is the most beautiful song I have ever heard)
Also: Greensleeves, Danny Boy, and Ashokan Farewell (from the Civil War soundtrack)
I don't just like classic rock/classical music/R&B, but those are more of what I consider the "greatest songs ever written".