Interplay - ballet - was created in 1945.

Chilean National Ballet was created in 1945.

Aaron Copland won the Pulitzer Prize for Music in 1945 for his ballet score "Appalachian Spring."

There is not one that is the best but there are many amazing ones. Canada's National Ballet School, American Ballet Theater, Royal Ballet School, Paris Opera, Australian Ballet School, Houston Ballet School, Stuttgart Ballet, and the list goes on....

I'm 90% sure there isn't an official list. But for international companies, they have what they call the "gold standard" list. This list includes includes the Kirov, Bolshoi, Paris Opera Ballet, Royal Ballet, Royal Danish Ballet, and New York City Ballet, in which case NYCB would clearly be among the top 15 in the US. But for the others, a list of that sort would most likely just cause a lot of arguing.