The very first shot of the show was Denholm, with his hands poised together and a really serious look on his face...revealed to be a photograph, underneath which he has the exact same pose!
"I hope this doesn't intimidate you, Jen, but I've found that the best way to greet a new employee is to size them up with a long, hard stare."
It may be a toss-up between "The Speech" in which Jen breaks the Internet, or "Are We Not Men?" which gives us the longest Fake-Out Make-Out in human history... between Roy and Moss, in order to hide from passing police cars (of which there are far too many), instead of using a collection of huge bins nearby as cover. And their all-purpose football dialogue. And Jen's magician boyfriend.
The "Could he have thought you were a man?" Brick Joke in The Work Outing. And that entire episode on top.
"Here we are. Your place... your place. La Mason de la femme, El casa de las senorita... DAS HOUSE DIE FRAU!"
This little conversation between Roy and his girlfriend in "Italian for Beginners": "What are you doing on the computer?" "Masturbating."
What makes that scene so awesome is the look on Roy's face, a "whew" that seems to say "Oh, she just thought I was borrowing her laptop to look up pictures of other women naked. I can't believe I got away without her knowing what I was really doing!"
Moss sending an e-mail to the fire department. "Fire! Fire! Fire! Looking forward to hearing from you."
"I'll just put this over here with the rest of the fire."
"Dear sir/madam, I am writing to inform you of a fire that has risen up in - no, that's too formal!"
"Tramps Like Us", where it starts with Roy getting spilled on him and Moss getting concussed and ends with a nun, a priest, a rabbi and Douglas's electric sex pants. Moss goes to his happy place...
Moss trying to start a bra company and going on Dragons' Den.
The Internet-in-a-box from "The Speech". "What's Jen doing with the Internet?"
"Has it been... DEMAGNETIZED??"
"C'mon, boys, the Elders of the Internet... know who I am?!"
Douglas's knuckle-down action movie-style fight with his transgender ex-girlfriend in "The Speech" may be a case of Refuge in Audacity or Unfortunate Implications (depending on your viewpoint), but it is also hilarious.
Take note of the fact that, during the fight, Douglas literally causes damage to company property (including a very unfortunate wall, whole shelves of scientific equipment). Also, whenever he's knocked down, he jumps up like a kangaroo (which is just hilarious).
There was a fire...in a Sea Parks?
"Why would she lie? And if she was going to lie, why would she use this one? A fire at a Sea Parks?! It's wrecking my head! I mean if... if she had said that her parents had drowned I'd be the happiest man in the world!"
"What are you doing on that computer" *Beat* "MASTURBATING".