Also have to go with "Dear Doctor." A truly repellent episode that uses racist pseudoscience to justify passive genocide. That's the episode that needs to be stricken from canon.
Several people have mentioned this episode but sorry I have the worst memory for titles. What was that one about?
a first season ENT episode where the crew comes to a planet with two races, the Valakians and the Menk, living together. One of the races is dying of a plague, and even though Phlox discovers a cure, he and Archer agree to withhold it on the basis of Phlox' devotion to racist pseudoscientific mysticism about the belief that the race not stricken by the plague was "destined by evolution" to be the superior race and inherit that planet, and that he can't interfere with that process.
It's an episode that draws heavily from discredited 19th and early 20th century conceptions of racial Social Darwinism and Eugenics. If the writers had bothered to do an hour's worth of internet research, they'd understand how wrong their understanding of evolution was.
The idea was also to show the "wisdom" of the Prime Directive before it was put in place, but it failed utterly on that, too.