aren't I?
Here's why:
* The statement is positive: "I am the best student" is a positive statement.
* The verb is "am": The verb in the statement is "am," which is the first-person singular form of the verb "to be."
* Question tags use the opposite form of the verb: The opposite form of "am" is "aren't."
Therefore, the complete sentence with the question tag is: "I am the best student, aren't I?"