I'm studying in B.Com final year from RCU University. I want go for MBA. Which specialisation is best and which entrance do I need to clear?

It depends on whether you want to stay in India or you want to go abroad. If you are planning to stay in India and you want to take admission here, then you have to appear for CAT, XAT, MAT, etc. Some colleges also have their separate entrance exams. Some of the very good colleges here additionally

CAT is the best exam for Management studies in India. There are also exams like CMAT, XAT, SNAP, etc. All fields are good for MBA, it is only a matter of your interest.