(THAT body part is only the third worst.)
Honeybee researcher Michael Smith, himself no stranger to bee stings, had a question without an answer: what is the most painful place on the body to be stung by a honeybee?
Since nobody else had yet been willing to step up, he had only one way to find out!
Over 38 days, he picked up bees with forceps and held them against various body parts (his own, at least partially to avoid running afoul of ethical rules and regulations governing human testing). Each angry bee stinger was left in for a full minute, with Smith rating his self-inflicted stings on a 1-10 scale. He also inflicted "calibrating" stings on his forearms before and after each session. Each tested body part, 25 in total, was stung three times over the course of the experiment. (He ultimately decided against stinging a 26th body part, his eyeball, after an adviser cautioned he might lose his vision.)
The worst place to be stung? The nostrils scored a 9.0 out of 10, followed by his upper lip (8.7) and penis (7.3). The best places to get stung were also generally the least likely to be visited by bees: the skull (usually covered by hair), the tip of his middle toe (usually covered by shoes), and his upper arm (yeah, bees could get to this one), earning only a 2.3 each.
You can find the full table of results, plus plenty of commentary on his methods and results, in his open-access paper Honey bee sting pain index by body location hosted at PeerJ.