Absolutely Invincible British Gentleman is my ultimate favourite, but the others are (in order):
Pub and GO!
Ren Ren Renaissance
Delicious Tomato Song
Excuse me I'm Sorry
Aiyaa Four Thousand Years
My Heart has a Light
Basically all of them. And Hamburger Street is awesome. Trust America to sing about burgers. The Nordic 5 is good too.
I think China's Hello China has become my favorite.
Followed by Aiyaa Four Thousand Years, and then Spain's, Britain's, and Japan's songs. Oh, and Mein Gott.
"My Heart Has a Light", "Winter", "Embrace The Tres Bien Moi", and "Paris Is a Splendid Place Indeed" are my favorites.