There are so many, but three come to mind:
3. From "In The Pale Moonlight":
SISKO: " I bribed men to cover the crimes of other men. I am an accessory to murder. But most damning thing 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."
2. From "Waltz":
SISKO: "I fear no evil. From now on, it's him or me!"
1. From "Call To Arms," the moment where the Defiant
and Martok's ship join the combined Federation/Klingon fleet--the first time we see a fleet of ships and an absolutely superb musical cue that sends tingles up the spine...