Since biblical times, nobody's liked tax collectors. :) good question Will... I'm not sure how I would answer!
I'd go with Teacher. They put up with so much (e.g. lack of funding) and in recent times, now you have parents that blame the teachers for their child's F rather than their son/daughter. Terrible.
Teachers are so gifted and not respected at all. They are truly something special. My vote is an ER nurse. My wife was an ER nurse and I have heard horror storys. Spitting, hitting, hate words. Are you kidding me? I mean they are helping people!
I believe that Sports Officials potentially receive fewer Thanks than most all careers. Not only is there a lack of Thanks, but they are berated quite often, if not constantly.
All the best,