Dexter's narration introducing himself while he's out on the boat and talking about his parents being dead, then swearing that he didn't kill them.
In Episode 3, Dexter is stalking a boy who committed murder in the past and he believes will kill again. The boy has convinced a friend to hunt alligators, and it looks like that was a ruse to lure him there and kill him. Dexter interupts this and then wonders what he's doing there (saving someone's life). At this point, the actual alligator lunges at Dexter causing him to fall in the mud. He then bangs his head on a tree.
Miami PD has discovered one of Dexter's victims courtesy of the Ice Truck Killer. Dex tries to convince Masuka that the injection site where he loaded up the victim with tranquilizers was a bug bite, but Masuka isn't buying it. He discovers tranquilizers, and a list of names of people who have access to it. Dex's (internal) response?
Dexter: That's it. No more donuts for Masuka.
Rita saying "I can't help it. I kill things", and Dexter giving her a sympathetic look.
Dexter postponing the torture of a couple to quiz them on how to form a loving relationship.
A romantic date with Rita watching Terms Of Endearment. We see Rita crying buckets as she watches, then looks at Dexter's completely unemotional face. He's wondering if he can make his eyes water by not blinking. He decides to break the silence by attempting oral sex on her.
Paul, Rita's abusive ex-husband, is attempting to legally remove Rita's custody of her children. Alone in the kitchen, Paul snidely threatens Dexter and Rita. Of course, we never get to hear him finish his threat, because Dexter promptly clocks him over the head with a frying pan. The real kicker to the scene is Dexter staring down at the body, cringing with the realization of what he just did. Of course, he finds a solution...
What makes it funny is that he doesn't cringe in a "What have I done?" kind of way, but rather in an "Oops!" kind of way.
In episode 7, he tries to psyche out Dexter by throwing a punch at him and stopping inches from his head, Dexter doesn't even flinch, Rita and Dexter talking about it after it is priceless.
Rita: Did he threaten you?
Dexter: No, he was just marking his territory.
Rita: Fucking bastard!
Dexter: Only you can make those words sound cute.
Dexter's admission to his temporary psychiatrist that he's a serial killer, followed by the nearly orgasmic noise and face he makes as the relief of finally saying that out loud washes over him, always draws a laugh. It really shouldn't.
Dexter: ...I'm a serial killer. [pause, then lets out a deep sigh] Oh God. That feels so amazing to say out loud [...] I'm not joking; I kill people. Whoo. There it is again.
Episode 9, Dexter and Rita end up going to Dexter's recently deceased biological dad's house and Rudy and Debra show up to help. Dexter's thoughts on the matter the next morning is priceless.
Dexter: When did a man dying become a couple's weekend?
Doakes: Surprise, motherfucker.
Also from Doakes when Dexter matches him pound for pound in a fight: "Lab geek, my ass!"
Dexter imagines himself a parade after killing the Ice Truck Killer.
"YEAH, DEXTER, PROTECTING OUR CHILDREN!"
Doakes and Deb having dinner at Doakes family's house, and how Doakes has to sit there and be polite while everyone else teases him and makes him uncomfortable.
The Ice Truck Killer wrapping body parts and singing "Deck the Halls." They're very festive.
Dexter: Lie like that? And I thought I was good, but you...[Gives an Italian "Bellissimo" gesture]
Hicks: I don't lie!
Dexter: ...OK, that one was weak.
Doakes: "You owe me a new Michelin, you motherfucker!" -Season 2 Ep. 3
Near the end of Season 2, when Doakes corners Dexter with one of his fresh kills, Dexter's deadpan response is the blackest of black humour:
Doakes: Jesus Christ, Morgan. Jesus fucking Christ, you're the Bay Harbour Butcher.
Dexter: [completely deadpan] I really hate that name.
When Rita confronts Dexter about attacking Paul, the mood is very tense and it seems as if Rita is about to find out that Dexter is a killer. She then does a complete 180* on our expectations and concludes that Dexter is a heroin addict.
Angel: You know, Pascal is just caught in a cycle of reaction. She's not co-creating her own reality.
Dexter: You know those words don't actually mean anything, right?
Cody has a nightmare about the Bay Harbor Butcher, Rita's response to it when she's talking to Dexter sells it.
Rita: [talking about the "Bay Harbor Butcher"] It makes me so angry that there's some monster out there scaring my son.
Dexter: [uncomfortable] I gotta go to work.
Rita: You know, I hope they catch him today. And I'm not a violent person, but I hope they'll hurt him. Have a good day. kisses Dexter goodbye]
Dexter: [voiceover] Make up your mind.
The Dark Defender....no, Miami's too hot for all that leather.
Dexter at rehab for the first time.
Dexter: No self control, lost everything, trying to stop; same whiny story over and over for …
[Dexter checks his watch.]
Dexter: Ten minutes? Feels like ten hours.
Addict: I'd kill for a Vicodin.
Dexter: [thoughts] Lightweight.
Deb's big shit-eating grin and wave at Lila when she tells her that Dexter lied about working late.
Roger Hicks sees Dexter checking out a minivan, and gives him the usual "family-friendly, kids love it" pitch. Dexter is about to walk away since all he needs is Hicks's DNA, until Hicks mentions the stow-away seats, giving the car a huge trunk with tinted windows. Cut to Dexter driving into Miami Metro with the minivan.
LaGuerta reveals to Dexter that she believes Miguel Prado killed Ellen Wolf, which of course Dexter is well aware of. Dexter's dramatic display of shock was stunning!
Dexter's pathetically short invite list for his wedding.
Rita: "Donut guy"?
Dexter: ...I see him every day.
In Episode 6, Dexter monologues that he prefers to kill alone "...present company excluded, of course."
That could be referring to the victims though.
During a meeting with Ellen Wolf:
Dexter: "Do I see sheets of plastic in your future?"
Dexter in yoga class with Rita is hilarious. "This is absolutely, without a doubt, the worst moment of my life."
Dexter is on his way to kill a paedophile. Rita calls and asks him to get milk. Dexter kills the paedophile in his home. While dragging the body across the kitchen, he pauses, opens the fridge. Takes the milk and goes back to dragging the body.
Dexter throwing a tantrum after finding out Miguel Prado had been using him. He completely wrecks his office and screams his supposed friend name in the most hammy way imaginable... only for Miguel to come out from the door, on which the screen shows all of that was just on his mind and Dexter didnt do anything.
After Dexter tells Masuka that he's testing a grey hair he found, Masuka claims that he's done the same thing:
Dexter: You're bald.
Masuka: *points to his crotch*
After a particularly tiring day, Dexter climbs into bed happy about how he'll finally get a good night's sleep. Cue Harrison crying.
While stalking the neighborhood vandal in an all-black outfit, he's mistaken for the vandal by the neighborhood watch. "This is embarrassing. It's 'observe and report,' you assholes, not 'observe and chase'!"
LaGuerta asks Dexter to come into her office so they can talk.
In Episode 7, Deb talks to Dexter after discovering he has had a long series of affairs, and Ghost Harry comments that disappointing your children is a father's worst nightmare. Dexter says that "it doesn't sound like Harry" and Ghost Harry turns to him and says, "Thank you, Dex!"
When Dexter is looking for an activity for Astor and Cody. "What did I like to do when I was 12?" (Beat)'' "Moving on."
Dexter makes a speech about weather effects on forensics to get LaGuerta to let him go to a convention in Tampa (as an alibi), and she agrees pretty much just to make him stop talking. Then, cut to Dexter using the same routine on Rita.
In the premiere, the redo of the opening credits sequence now that Dexter is put off his game by having a kid: he misses the mosquito, his shirt has a puke stain, and he breaks his shoelace. Thankfully it's interrupted before we see how much he can screw up juicing an orange.
After having to open his secret trunk in his secret apartment for a suspicious Rita to see, and Harry's shotgun is revealed to be inside:
Dexter: "You see? I've got nothing to hide from you."
Narrating Dexter: "Except for the syringes, scalpels, and bone saw in that secret drawer underneath."
"Is that Justin Bieber? Not that i know who he is" from Masuka and Quinn deadpan looking on him.
Masuka's explanation of auto-erotic mummification, complete with hand gestures.
From the same scene, Deb's "What in the name of fuck went on here?". The deadpan delivery and the absurdity of the situation just makes it perfect.
For that matter, the scene itself. The audience can miss it since they saw the chain of events leading to it but just imagine being one of the officers who were first to come. It's sheer Mind Fuck.
The bit where Dexter's monologue is going and the audio of Masuka's explanation is muted and you see him making exaggerated masturbation gestures while the EMTs and Deb just watch sends me into fits of laughter every time I watch that episode.
When discussing a possible ritual murder:
Deb: "It's about that cult, Santa Mierda or something."
Angel *laughs* "It's Santa Muerte, Morgan. It's Saint Death, not Saint Shit."
Dexter walks in on Deb and Harrison's babysitter having a heated argument, and wonders "Would it be weird if I just left?"
Dexter's confusion over Deb calling him a chair.
Quinn assumes he left his gun in the car of a stripper he picked up. It turns out to actually be the car of a middle-aged woman who works in a waffle house across the street. And they took pictures.
In "Run" Dexter hops around like a monkey while yelling "Fuuuuuck!" to taunt his F-bomb dropping victim.
Dexter and Debra talking in his car after Dexter captures Estrada for the second time. When Deb hears a thump from the trunk, Dexter admits he's hiding Estrada back there. Deb just gives an exasperated sigh and leaves the car.
After 5 seasons, we hear that famous phrase yet again, "Surprise motherfucka!"
In a flashback with Doakes, Dexter claims a friend told him the phrase he had just said. Doakes, without any delay, replies:
Doakes: Bullshit. You don't have any friends.
LaGuerta is determined to prove that Doakes wasn't the Bay Harbor Butcher, but a skeptical Matthews counters her with some rather off-color phrasing.
LaGuerta: We don't know Doakes used this place. You're jumping to conclusions.
Matthews: This address came from the tackle box, which came from the cabin where Doakes died. This isn't jumping to a conclusion—this conclusion's hanging from your ass.
*LaGuerta gives him a look of disgust*
LaGuerta: This evidence could have been planted. Dexter is a forensics expert, and he has the skills—
Matthews: Maria, it's over. The fat lady isn't singing. Her song is done. She's getting dry-humped in her dressing room.
In the season premier, Dexter logs into Debra's bank account to gain a location to where she had been hiding but needs her password. He tries "Password" but it doesn't work... when he writes "Fucking Password", he gains access. But Michael's deadpan of "Fucking Password" was priceless.
The look on Dexter's face◊ when he finds a cannibalized finger in some stew. It is notably one of the few times (if not the only time) that something has genuinely grossed him out.
After a particularity heated discussion between Debra and Dr. Vogel:
Deb: Why the fuck should I trust you?! Why the mother fuck do you care?!
Book Dexter is quite the Deadpan Snarker and as such is a good source of these. Early in Darkly Dreaming Dexter there's a scene where Dexter is dumping pieces of a body in the ocean and comments that people often mistake him for a fisherman: