Dexter/Funny

Two words:

Doakes: Surprise motherfucker.

Both funny and horrifying.

Sally: I'm so thankful for you, Arthur.
Arthur/Trinity: Shut up, cunt

Dexter keeping calm while having dinner with Rita's mother by pretending she's an alien.
Season 4, 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!"
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.

Rita saying "I can't help it. I kill things", and Dexter giving her a sympathetic look.
Dexter's pathetically short invite list for his wedding.

Rita: "Donut guy"?
Dexter: ...I see him every day.

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 decides to breaks 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 suggests to Dexter that Rita doesn't appreciate how dangerous this situation is, and that he'd do anything for his children....and if Rita or Dexter gets in his way....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...
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.

"What is it with you and rape? No one's raping anyone!"
In season 2, 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.
But the best is and will always be the montage of Dexter admitting he's the Butcher to Deb in season 2.
Dexter's conversation with Roger Hicks before killing him.

Dexter: Why can't I do that?
Hicks: Do what?
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.

Dexter in yoga class with Rita is hilarious. "This is absolutely, without a doubt, the worst moment of my life."
The moment in Season 2, when Dexter and Deb are roommates and getting on each other's nerves. Dexter's voiceover: "I will not kill my sister, I will not kill my sister, I will not kill my sister....." From anyone else, meh. From him, hilarious.
A while ago, there was an officially approved joke comic (by Scott Allie and Tim Seeley) floating around the internet. In it, Dexter, syringe in hand, is shown attacking a hulking killer... who turns out to be Jason Voorhees. Cue Megaton Punch.
In episode 3 of the first season, 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.
In the season 4 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.
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!"
Dexter's double-knock-out with Boyd. Even better when they wake up in the ambulance and begin lying about what happened without contradicting each other.
- Also, Dexter's "Oh shit....".
When discussing a possible ritual murder in season 5:

Deb: "It's about that cult, Santa Mierda or something."
Angel *laughs* "It's Santa Muerte, Morgan. It's Saint Death, not Saint Shit."

Masuka's tattoo.
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 the police start to piece together what the Trinity Killer must look like:

Angel: So, we're looking for an old, retired, white guy...in Miami?

"Do I see plastic sheets in your future?"
"Just a cat that hiccups and farts at the same time." "Cute."
Season 1, 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.

In season 2, Dexter has a period where he behaves almost sane. The narration stops for about an episode and a half. At one point, things take a turn for the worse, and the narration begins again, with this gem:

Dexter: "I need to delete that file, make up with Rita, and while I'm at it, I may as well end world hunger, too, since this can't get much more difficult. And the voices are back! Great.

- Another where his inner monologue is saying something to the effect that he prefers to kill alone "present company excluded, of course."
- That could be referring to the victims though.
After having to open his secret trunk in his secret apartment for a suspicious Rita to see in season four, 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."

Hey Dex, where you going?

Narrating Dexter: "Kill the guy you're looking for, save a kid from a serial killer...remember to pick up diapers."

Dexter imagines himself a parade after killing the Ice Truck Killer.

"YEAH, DEXTER, PROTECTING OUR CHILDREN!"

Deb: She's like a Gross, English... Titty... Vampire!
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.
In season 2, a stressed-out Masuka complains to Dexter that the Bay Harbor Butcher is "butchering my social life!" He then leaves the room. Dexter's response? A soft "Sorry."
After Deb walks into the apartment and finds a naked Lila raiding the fridge:

Deb: Who the fuck are you?
Lila: Hi, I'm Lila. *looks down, remembers she's topless* Pardon my tits.

LaGuerta asks Dexter to come into her office so they can talk.

LaGuerta: I know everything, Dexter.
Dexter (voice over): I'm gonna choose not to misinterpret that.

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*
Dexter: ...Oh.

In Season 3, LaGuerta revealed to Dexter that she believed Miguel Prado killed Ellen Wolf, which of course he was well-aware of by then. Dexter's dramatic display of shock was stunning!
After a particularly tiring day, Dexter climbs into bed happy about how he'll finally get a good nights sleep. Cue Harrison crying.

Dexter: Fuck.

Be careful what you mime, Debra.
In the Season 6 premier, Dexter attempting to dance to MC Hammer with Harry giving him the double thumbs-up. But the best part is the voice-over.

Dexter: I have no idea what Hammertime is. Or how it differs from regular time.

- Dexter's horrified realization that he's popular now.
- Angel freaking out that people will think his sister is his girlfriend.

"I said you weren't my girlfriend. Now she's going to think you're a hooker."

- The Call Back to Hammertime, in the only way Dexter knows how.
- Voice-over Dexter's response to Dexter's babbling attempts to stop a hot ex-classmate giving him a surprise blow job:

"Dexter, shut up"

After Quinn proposes to Deb:

Dexter: Debra Quinn. Your initials would be DQ. Like the...

Deb: I will kill you.

Deb discovers pantyhose:

Deb: Has anyone ever died from crotch asphyxiation?
Dexter: You have officially crossed the line from appropriate brother sister conversation.

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.
How about this gem from Season 6? Vince's new intern has just perfected a video game where you play as famous serial killers throughout history:

Intern: You can be Dahmer...or Jack the Ripper...or the Bay Harbor Butcher...
Dexter's inner monologue: [with hilarious indignation] I am the Bay Harbor Butcher.

In Season 4, when Dexter is looking for an activity for Astor and Cody. "What did I like to do when I was 12?" (Beat) "Moving on."