Sisko's last monologue at the end of In the Pale Moonlight. If there was any doubt that DS9 was not playing by the same rules as any of the other Trek series, this episode pushed that doubt away.
"So... I lied. I cheated. I bribed men to cover up the crimes of other men. I am an accessory to murder. But most damning of all... I think I can live with it... And if I had to do it all over again... I would. Garak was right about one thing – a guilty conscience is a small price to pay for the safety of the Alpha Quadrant. So I will learn to live with it...Because I can live with it...I can live with it. Computer – erase that entire personal log."
I came here to post this.
Ending of The Visitor made me cry like a little bitch though.