I would be interested in hearing of any country with the sheer volume of immigrants that the US has from every continent. We have migrants from every European nation, africa (including modern african culture, not just the results of the slave trade and imancipation), asian immigrants, including China, Japan, Veitnamese, Indian, even indigenous peoples as far as Oceania. And every culture is represented in our own, much to the dismay of those that believe our great works came solely from white hands. Our democracy is founded on a melting pot of ideas and inputs, not a euro-centric empire graciously allowing "lesser men" in it's borders.
It looks like the States is still one of more diverse of the 1st world countries. So I guess that's something. It's tough to beat Africa at that statistic
It depends on if you are speaking about ethnicity, the concept of race, or the diversity of genetics. I guess it also depends if you are talking about the percent of the total population, or just the total amount of ethnic groups living in the state.
America may be racially diverse, or have a large number of different ethnicities living in the country. But when it comes to ethnic and genetic diversity, it's pretty hard to beat just about any country in Africa.