In Thief 2, enjoy the escalating argument between two groups of guards defending their manor lord/lady. The insults they come up with are hilarious.
Guard 1: And I'm telling you that the only stench heartier than your rotting burrick of a master, is the liquor on his fetid breath. If he comes near Lady van Vernon again, we'll boil his knackers.
Guard 2: Oh, ho! Mighty fine words coming from a knock-kneed, inbred pageboy such as yourself. Our good master Willy wouldn't be caught near that frumpy little trollop unless he were holding her back at the end of a halberd.
Guard 1:(shocked gasp) How dare you defile the name of someone so good and virtuous as the Lady van Vernon. Our Lady is a saint among mortal women. An angel so pure the heavens couldn't hold her.
Guard 2:(scoffs) Hahaha! Your Lady? An angel?! You're lucky the dockside whorekeeps aren't bashing down her door for stealing their clientele. Why just last night, I saw her out back warming up the stable-boy.
Guard 1: Such slander will not be tolerated while we're on watch! You'd best run and rescue your helpless limp lord before he flounders in his own vomit, or wakes up naked in a hen coop. Scurry off, or you'll acquire some unnecessary ventilation!
Guard 2: Is that a threat, you shriveled old maid? You gonna prick us with your sewing needles? On this side of the street we shoot like soldiers, so don't make promises your arrows can't keep.
Guard 1: (voice cracks) YOU'VE GONE — you've gone too far this time, you camel-mannered, tunic-wearing mollycoddle! An arrow in the throat ought to shut you up!!
Oil flasks and moss arrows in Deadly Shadows. While moss arrows always gagged guards in the series, the sound and action of them gagging is particularly funny in the third game. Of course, watching guards take pratfalls on the slippery oil is funny, but it's even more funny when they do it right off a pier in the dock. Thanks to their Super Drowning Skills, it's a useful tactic, too.
Garrett and his contempt for his fellow criminals.
Garrett (In The Metal Age): ...Homebreakers. How quaint.
Garrett (In Deadly Shadows, snarking over a Costume Copycat): So, that's the famous Garrett. Guess he's not as good-looking as I heard.
The first game's mission Return to the Cathedral, being the forerunner of Robbing the Cradle from the third game, is terrifying, but there's one moment of hilarity: Garrett reading aloud a Hammerite prayer in a tone of utter blistering sarcasm. Stephen Russell nails the total boredom in his voice.
Raoul, the crazy hermit who used to own the opera house in the "Song of the Caverns" level of Thief Gold.
(notices Garrett) "What's this? A small fish? Oh... no, it's a big fish! Have you come to sing with me? Come, come! Let us re-enact the dramatic scene between Raoul and the Lady Valerius!"
Near the end of the third game, in the middle of a very dire situation, there is one moment of humor. As the Hag appears before Garrett and Orland, she kills Orland while bragging about how awesome her power is... and Garrett just kind of nonchalantly walks away and disappears into the shadows. It's a good twenty seconds before the Hag notices he isn't there any more and she's just talking to herself.
Garrett decides to work with the Keepers in Deadly Shadows, and expresses his exasperation with the vague prophecies delivered by Caduca and her translator, Gamall. Afterward, Head Keeper Orland makes it very clear that he doesn't trust Garrett, and warns him that if anything goes missing from the Keeper Compound, he will be holding Garrett responsible. Garrett just smirks at him.
Orland: Is that clear?
Garrett: Perfectly. Maybe you should translate for Caduca.
The "kick me" note in the police station in Thief 2.
During the ending of Deadly Shadows:
Garrett: If I never see another glyph again, it'll be too soon.