I underwent surgery which I found out later was unnecessary. Do I have a medical malpractice claim?
Possibly, if there was no legitimate medical need for the operation, or if your consent was procured by fraud, or if the surgical operation departed from standard medical practice.
The choice of treatment often is a "judgment call" and competent physicians can reasonably disagree on choices of treatment. The doctor using his or her professional judgment chose surgery as the method of treatment, subject to your informed consent.
The critical question of liability, however, will focus on whether the operating surgeon was negligent and did not perform the operation in conformity with standard practice, not on whether other reputable professionals would have recommended other treatment. Whether or not you would have a solid malpractice case depends quite a bit on the facts in your particular situation. Consult an experienced attorney in your area to determine if, in fact, you have a good case.