Which country is the best in terms of opportunities for maths graduates?
Depends what your metrics are and how good a graduate you are.
If your main consideration is money then it actually doesn't matter much because all the hyper-money is in firms which are multinational traders. Around the top would be 'quants' - quantitative analysts - working in the city to predict market movements. You need to be at the top of the class to even get a look in, but the salaries are silly money - millions in a bad year.
I think the best opportunities will be found in USA.
It is a big country and it offers more oppurtunities than UK or German.