J-pop is where it's at, btw, but here are my own favorites:

Utada Hikaru - Keep Tryin'
Village People - YMCA
Moldy Peaches - Anyone Else
Wicked - Popular
Freezepop -Science Genius Girl
Uptown Girl - Billy Joel
Lily Allen - LDN
Regina Spektor - On The Radio
Smile.dk - Butterfly

Anything by Morning Musume, especially THE MANPOWER!!!, Jyosei Kashimashi Monogatari, and Koi no Dance Site (SEXYBEAMMM!!!!)

And even though no one will believe me, I think Rick Astley - Never Gonna Give You Up is one of the most upbeat songs ever.