hello aspirant,
There are many post graduate courses like M.tech and M.sc and phd.
also,  you can go for m.phil i e master of philosophy which has highest demand and more career options.
also, i will tell you the specializations of post graduate programs after your b.sc maths
1)Master of arts in mathematics
2)master of philosophy in mathematics
3)master of science in bio statistics
4)master of science in applied mathematics
5)master of science in pure mathematics
6)PH.d in math Education
7)pure mathematics masters program
you can go for any of those above mentioned courses which are having a great demand in current trends.
The best universities regarding that courses are mentioned below:
1) christ university
2)university of delhi
3) IISC, BANGLORE
4)university of calcutta
5)harvard university
Since we cannot rank the best universities because every university is very very best in their specializations aspects.
hope it helps for you,
thank you