When I read the question initially, my first thought was my breasts. Of all the components of the feminine anatomy that is the one aspect that I feel that I am missing. However, after reading some of the other responses, I realized that I might have to give question additional consideration.
I have to agree with several other responses that my hands and feet should also be given adequate examination. I say my hands, not because of their larger size or manly appearance, both of which are true, but actually due to their hair growth. I have always felt the hair growing on my hands is somehow misplaced. As for my feet, it would be their size. I do find it ironic that all my later childhood, I had hoped for feet on the larger size, but now as a adult, I wish they were just 2 sizes smaller.
However, giving all due consideration, I must return to my original thought. Of all of who I am, it is my lack of breasts that I find the most wrong. Over the years, I have come to realize that I strongly believe that I should have my own natural breasts. I do understand that man (something that I unequivocally know that I am and should be) having breasts is not typical, but it is something that I deeply feel is part of the complete and true me.