I'm guessing the best countries for an MBA and further career in IB would simply be the most developed ones, but are there any that especially excel? I'm guessing in Europe it'd be Switzerland and Germany, and outside of it maybe Singapore and the US? Most of the top schools for MBAs in investment banking seem to be in the US, such as Wharton and NYU Stern. Very few from outside are ever mentioned.

Best in what sense? IB jobs have been moving from the West to Asia for 30 years. The most valuable roles as client facing, but the opportunities are where the hiring is. Whatever you want to know about IB, Google is a  true friend.

thanks!