Rita and Dexter both have individual moments with the same purpose in Season 1: Both of them attack Paul, Rita in self-defense and Dexter so that he can frame him to protect Rita.
Rita forcing her abusive ex-husband to sign the divorce papers in exchange for supervised visits with the kids, reminding him that she has the power to make him never be able to see his kids again.
In "Born Free," the last episode of the first season: the Ice Truck Killer having been revealed as Dexter's long-lost brother, creeps into Dexter's apartment with the intent of killing Dexter's foster sister. As he pulls back the covers to discover he's just stabbed a mannequin, Dexter looms up behind him and loops a garrote around his throat.
One of the earlier episodes of season two. Dexter after a car salesman that deals with his misogyny through murder, and Dexter wants advice on telling lies. The salesman gets into a rant about women, and calls Rita (Dexter's girlfriend) the c-word.
Dexter: DON'T...talk that way about my girlfriend. (stabs the hell out of the salesman)
Dexter in Season 2 when Doakes confronts him about having something to hide,and Dexter states that he'll always be one step ahead of Doakes. When Doakes questions this, Dexter gets into his face and says "I own you" and headbutts him in the face, leaving his office to let Doakes chase him down and make a scene in the middle of the station.Watch it here.
Lundy realizing a letter from the serial killer has been fabricated to provide all sorts of false leads, and quickly deducing the perpetrator is so savvy that he has to be one of their own: "Law enforcement"
Rita finally telling her mother to shut up and tells her to get out and only show up for holidays and birthdays after her mother has constantly criticized Rita over so many things.
Dexter overpowering Doakes while handcuffed, in the water, with a wounded leg
Dexter breaking his own hand to escape from the man about to torture him then killing said man and throwing him in front of a police car in the Season 3 finale.
Made even more awesome by the fact that Dexter pulls a successful Hannibal Lecture on the Skinner that makes him lose the control that he wanted a few seconds before the awesome events mentioned above.
Season 3 Big Bad Miguel Prado gets one when he's not even present: Dexter discovers the supposedly incriminating blood-soaked shirt Miguel gave him as a sign of trust only contains cow blood, indicating that everything that's happened so far has been an Evil Plan by him.
He has another one a few episodes earlier, where he orchestrates the death of an Aryan Brotherhood gang leader and monster that Dexter had been unable to get at for years.
"Science is one cold-hearted bitch with a 14-inch strapon." Masuka's moment of awesome while shooting down someone trying to sneak something past him in a report.
Deb's pushing Dexter into accepting his responsibility in being a father for his unborn child.
Dexter in Season 4 when Arthur almost strangles his own son in front of Dexter and Dexter uses his belt to choke Arthur, drags him to the kitchen, throws him to the floor, and, grabbing a knife, jumps on top of him saying "I should have killed you when I had the chance."
Also Jonah himself: what causes his father to try to kill him is his finally Calling the Old Man Out, culminating in his destroying Arthur's treasured urn of his sister's ashes.
Also from season 4, Dexter rescuing Scott Smith from a cement grave just in the nick of time, after whacking Trinity with his own shovel.
Kyle Butler and his cover get blown on his own workplace by Trinity, who casually drops an ominous line.
Trinity: Hello... Dexter Morgan.
In the season five premiere, Dexter knowingly takes an innocent life for the first time. Yet we still cheer him on since the guy was still a gigantic Jerkass who'd just viciously insulted Rita.
The thing about this kill was that it was entirely motivated by human emotion, Dexter's incredible grief and pain at losing Rita. So in twisted way, despite committing a brutal murder, it was Dexter being more human than he'd ever been.
Dexter: Because, this will clear you of being suspected, and leave you to me. And to her. Tick tick tick, your time's up.
Deb shooting Carlos Fuentes when he takes a hostage, after her previous hesitation with him almost got a hostage killed.
Dexter in season 5 killing Liddy, shortly after being tazered and starting with his hands tied behind his back.
In the last episode of season 5 Dexter stabbing Chase in the foot pinning him to the ground and everything that follows shortly.
What truly makes it is Chase's Oh Crap beforehand as he realizes one of the knives is missing.
Dexter beating the crap out of Olivia's step dad for hitting her in secret. Even Harry thought it was awesome.
Made so much more awesome by how calm he remains throughout
By the way, that's your kidney.
Deb taking down a man shooting up a restaurant, shooting his legs when she realizes he has a bulletproof vest.
Angel and Quinn finding the Doomsday Killer's identity while stoned off their asses, completely showing up that same new detective from above.
Dexter killing Nick on the spot with far more passion than usual, after he shows no remorse for killing Brother Sam.
Dexter banishing Brian from his subconscious, after realizing he truly does have a conscience now, doing so by running over his ghost. It's just made perfect when he then picks up a hitch-hiking Harry.
In "Talk to the Hand", Dexter saves the police department from a poison gas attack, even risking his own life so everyone can get out in time.