***** NOT A TEACHER *****
Hello, Ali Ahmadi:
1. I am your best friend, aren't I?
2. I am your best, am I not?
In ordinary conversation, it would be a good idea, I think, to use #1.
Number 2 would be fine if: (a) You wanted to be more formal or (b) you wanted to be emphatic (perhaps you are angry).
James: May I borrow $100?
James: What do you mean "No"? I am your best friend, am I not? Have I not lent you money in the past? (Instead of "Haven't I lent you money in the past?")
Student or Learner