America: I think it goes without saying that you have given us countless modern inventions that the world can be grateful for. If I have to be specific though, then probably the film industry. Also popcorn chicken...
You also have thousands of artists, inventors, writers, musicians, actors and comedians to be proud of.
China: A beautiful country with an incredibly rich history and some amazing architecture. But more importantly, Britain would be lost without delicious Chinese take-away. Your martial arts and fireworks are both pretty badass too.
Japan: Please don't ever stop being batshit insane, Japan. Your culture can be so imaginative and beautiful too and we love you for it. You also gave us the world of video games which is a pretty big deal in these forums.
I'll do some more later.