Currently, this song is "Rolling in the Deep" by Adele (even though the song itself isn't really happy.)
But of course, there are some others:
-Lady Gaga "Just Dance"
It's really hard to remember the days when Stefani was just eccentric and not a complete looney tune.
-Arcade Fire "We Used to Wait"
-Against Me! "Pints of Guinness Make You Strong"
-LuLu "To Sir with Love"
-Daft Punk "Digital Love"
-Edward Sharpe & the Magnetic Zeros "Home"
-The Beatles "Two of Us"
-Modest Mouse "Float On"