Way of the Warrior, Parts I & II (4x01): Hey, it's that Worf guy. Way of the Warrior is the first episode to get those big, epic fights DS9 becomes known for. The buildup to the fight and the fight itself at the end do a great job of keeping everyone busy, working characters, and keeping the action going. It is the best fight scene in Trek, hands-down. That title is sort of like winning a snail race, but it's still a win.
It's a repilot that clearly marks the point where the show hits its stride. This isn't a ratty, abandoned station with a crew that's trying not to kill each other. This isn't three runabouts, a commander, and two-dozen Starfleet officers thrown at a floating sprocket with a vague missive about "not fucking it up" that's faintly heard over the sound of an admiral's ship entering warp. This isn't those same officers bitching about being "in the middle of nowhere;" it's their home.
This is Deep Space Nine, commanded by Captain Ben Sisko, home port of the USS Defiant, Gateway to The Celestial Temple, the quadrant's first line of defense against The Dominion, and an ass-kicking dispensary open 26/7, willing to serve Klingons, Cardassians, Jem'Hadar, Romulans, and Breen free of charge.
Look, you are either in or out at this point dude.