Might as well have the lyrics here. It's a fine example of that genre Jokes turned into songs. Lyrics copied from The Baseball Almanac:

THE GREATEST
Don Schlitz

Little boy in a baseball hat,
Stands in the field with his ball and bat,
Says, "I am the greatest player of them all,"
Puts his bat on his shoulder, and he tosses up his ball.

And the ball goes up and the ball comes down,
Swings his bat all the way around.
The world's so still you can hear the sound.
The baseball falls to the ground.

Now the little boy doesn't say a word,
Picks up his ball. He is undeterred,
Says, "I am the greatest there has ever been,"
And he grits his teeth and he tries it again.

And the ball goes up and the ball comes down,
Swings his bat all the way around.
The world's so still you can hear the sound.
The baseball falls to the ground.

He makes no excuses. He shows no fears.
He just closes his eyes and listens to the cheers.

Little boy, he adjusts his hat,
Picks up his ball, stares at his bat,
Says, "I am the greatest." The game is on the line,
And he gives his all one last time.

And the ball goes up like the moon so bright,
Swings his bat with all his might,
And the world's so still as still can be,
And the baseball falls, and that's strike three.

Now it's suppertime and his mama calls.
Little boy starts home with his bat and ball,
Says, "I am the greatest. That is a fact,
But even I didn't know I could pitch like that."

[Recorded by Kenny Rogers on "She Rides Wild Horses, 1999 (plus several later albums), and by Don Schlitz on "Live at the Bluebird Café," 2001.]