"Meridian," the part with the erotic holosuite program where the head on Kira's hologram is replaced with Quark....
...JUST KIDDING...
Several of my favorites were in the epidode, "Inter Arma Enim Silent Leges." I think one that stands out for me is Sloan's dialogue with Bashir.
SLOAN: For being a decent human being. That's why we selected you in the first place, Doctor. We needed somebody who wanted to play the game, but who would only go so far. When the time came, you stood your ground. You did the right thing. You reached out to an enemy, you told her the truth, you tried to stop a murder. The Federation needs men like you, Doctor. Men of conscience, men of principle, men who can sleep at night. You're also the reason Section Thirty one exists. Someone has to protect men like you from a universe that doesn't share your sense of right and wrong.
BASHIR: Should I feel sorry for you? Should I be weeping over the burden you're forced to carry in order to protect the rest of us?
SLOAN: It is an honour to know you, Doctor. Goodnight.