I know there's no best song, but if I had to pick one song...
Most popular songs are based on a few standard chord progressions. The most basic is the 1-4-5, also known as the standard blues progression or 12-bar blues. You've heard it a million times. Whatever chord you start on, you go up to the fourth note and play that chord, then back to the first chord.
The notes in the key of C are C D E F G A B C, so C is the 1st, F the 4th, G the 5th. To "hear" these notes, consider the do-re-mi scale used in Western music. Sing 'do', think of 're' and mi', then sing "fa" (the 4th note). Try singing "DO do-do do-do do-do" (with the first 'DO' twice as long), then "FA fa-fa fa-fa fa-fa", and back to "DO do-do do-do do-do"
There's something about the song Hallelujah
by Leonard Cohen that resonates with me. Several other singers have performed the song. My favorite version that combines rawness with purity is done by Brandi Carlile (the iTunes store recommended it, so I listened to an excerpt and bought it).
One verse is "It goes like this: the 4th, the 5th," and those words "the 4th, the 5th" are sung on the 4th and 5th chords. Can you hear it going higher as those lyrics are sung? And how can you not appreciate a song that would rhyme "Hallelujah" with "what's it to ya?"
I guess I like it partly because it's a song about songs, as Hamlet is a play about plays and The Player is a movie about movies.
The best movie might be "It's a Wonderful Life
." It was painfully corny in 1948 and is no less so today. But why do so many people cherish this movie so?
Everyone's beliefs may differ, but I think the movie resonates with many people because it coincides with some spiritual reality (that our struggles are known) or maybe just coincides with the way we'd like it to be. I watch it just about every year around Christmas and still enjoy it.
Course if you chose "Seven Samurai" I wouldn't argue.