The US, Russia, China, and India are all large countries with hundreds of ethnic minorities. I think it's hard to tell whether one is more diverse than the others, but they are probably the top four. People tend not to think of China as being culturally diverse, but it is as big a melting pot as the other three.
I was thinking the US as well, I have traveled to a few other countries and have lived in Germany and Honduras. Honduras is not that divers and Germany I meet very few people at even the fest that where from other countries.
Well, it's hard to really say one in particular is more diverse than the other (with a few obvious exceptions), but if you look at it as a "per capita" number, then I'd have to say probably the US
South Africa is also extremely culturally diversified, but if I have to say the most I'd go with india. There are tons of languages, people and cultures in there.
Australia is the most culturally diversified country in the world. Most kids have parents or grandparents or great grandparents who immigrated to Australia.
Thats a tough question. I would guess Canada and the States.