Deep Space Nine "Extreme Measures" and it really pissed me off.
Bashir spends the whole episode talking about how evil 31 is and how the ends don't justify the means, then he essentially becomes just like 31. He lies to Starfleet, he pretty much kills Sloan, then mind-rapes him, then pats himself on the back over darts and drinks with O'Brien, completely oblivious to the repercussions of what he has done.
It's enough to make me re-evaluate the character of Bashir as a whole and the more I think about it, the more I dislike him.