The happiest song: "Cliffs of Dover" by Eric Johnson
It's so upbeat and in a great major key. Gotta love it.

Anything from Andrew WK's first album. Even "Get Ready to Die."

Jimmy Soul - If You Wanna Be Happy
I can't listen to the song without laughing.

<i>Saw your wife the other day.
Yeah?
Yeah, she's ugly.
Yeah, she's ugly but she sure can cook.
Yeah?. Okay.<i>

The Egg and I - Cowboy Bebop

If I were to choose one already listed, it'd probably be "Island In The Sun".

But here is my vote: "The Other Side" by the Dismemberment Plan.

Damn it feels good to be a gangsta - The Ghetto Boys
Don't worry, be happy by the whoever's... its annoying as hell, but its got be happy in the title...