Do not get a letter from:
An allied health professional (ie, PA, Nurse, PT, OT, etc.)
Your clergyman
A government official
A relative
A resident physician (loose rule, but you really want one from the highest ranking physician)
A physician who really doesn't know you. Some schools, esp. Osteo ones do want a letter from a physician, but if you have the choice make sure they actually have something valuable to say.
A non-science professor (loose rule and obviously to be broken if a school requires a letter from one
Bottom line is that a letter from a PA for acceptance to medical school will not carry much weight, no matter how well that person knows you or what it takes to be a physician/medical student.