I put None of the Above because you did say "the most," but we actually have had a pretty hott Teacher/Student scenario here and there. Doctors creep me out, Lawyers irritate me, and the rest just don't do much for me.
None of them really jump out at me as being much better than the others. For me it's the story that we develop around the role playing that makes it fun, not the specific role being played.
I'm not interested in those kind of role play fantasies. He has a unique disdain for authority so I can't imagine him agreeing to it in the first place.
I have had countless crushes on teachers in the past so that probably takes the lead here, but I also love fireman and policeman.
I hate the doctor's office/hospitals, so that's a no go. Lawyers, not so much the profession itself I guess, but I do enjoy a man or woman in a nice suit.
Hope this doesn't sound bad but I'm less turned on by the idea of a woman in a power position like those above. Much more into a student/cheerleader. Probably because I'd rather be the one taking charge.