in a nutshell:
(best) psychiatrist, psychologist, therapist, counselor (worst)
less short version:
psychiatrist: can rx meds, usually psych meds though from scope of practice, can diagnosis other issues, can make hrt and surgery letters
psychologist: no meds here, can make hrt and surgery letters, can diagnosis other issues
therapist: no meds, can make hrt and usually surgery letters depending on credentials and surgeon requirements, not suppose to diagnosis, but may do so
counselor: therapy only, might make a hrt letter, can't make a surgery letter, can't diagnosis
hope that helped
edit: in a way the best one is one you feel free to open up and talk to freely, the letters can be obtained when its time
(but for letters, they need to be licensed to practice therapy for best results in shortest time, assuming they know their practice well, and what you tell them is truthful)