Splicer: I made a promise to do no harm... and I mostly meant it.
While many of the Splicer rants across both games are great examples, this example from Doctor Grossman has got to be one of the best:
Dr. Grossman: "THERE'S SEMEN ON EVERYTHING! EVERYTHING!"
Dr Grossman: "When is a fever not a fever? WHEN I SAY IT ISN'T!"
When you first get the Electro-Bolt plasmid in the first game, it's a very scary scene. However, the mood is lightened when Atlas says "Steady now! Your genetic code is being rewritten! Just hold on, and everything will be fine!" Dark humor on the level of Portal, but also creepy in how normal that kind of thing is to Rapture citizens.
Waders: "He offered you salvation, and you SHIT on him!"
When you use the Enrage plasmid to make enemies fight each other, it's always a gas, especially when they're angrily cursing each other out. Also funny if you use it to make a lone splicer lash out at a Big Daddy. Hilarity Ensues.
When you're going after Hector Rodriguez for Cohen (the Nitro Splicer who continuously runs from you), make use of the Hypnotize Big Daddy plasmid and watch the thing go thundering down the hallways after him. It'll be a hilarious chase that will go halfway across Fort Frolic before Rodriguez finally gets beaten down.
When Fontaine realizes that "would you kindly" isn't effective at controlling you anymore, after asking you in vain two or three times to "Go get stepped on by a Big Daddy, would you kindly?" Something about the increasingly confused and irritated tone of his voice was just hilarious. It then stopped being funny and became really unpleasant when he reveals he knows another code phrase. One that gradually stops your heart.
Julie Langford's vocal impressions of people in her audio diaries are quite hilarious. Listen to her deep-voiced impression of Andrew Ryan, and the classic "Harness the flame, harness the mist!", and try not to laugh. It's not easy.
Julie Langford is voiced by the same woman who voices the "Ladysmith" Splicers. The woman is a master of having every word out of her mouth sound like it has air quotes around it.
It rarely happens and it's hard to catch, but when you alarm a Big Daddy that is currently escorting a Little Sister, he'll hoist her over his shoulder, and in the process, the Little Sister will yell out a joyous "WEEEE!" as she's flung over in the most adorable manner.
The game has numerous large, open-ended levels, each filled with a great deal of missions, but even more filled with hidden passages, ammo reserves, and security bots to reprogram. There's a lot to play with, and you're almost guaranteed to find your own humorous way to mess with the prowling Splicers.
One fun trick when being engaged by Splicers is standing behind a Big Daddy, at which point the Splicers will then try to shoot youthroughthe Big Daddy. Sit back and watch the carnage.
The "Cheap Son of a Bitch" audio diary by Suchong. "Go suck egg" is basically Suchong's hilarious version of "Go fuck yourself."
Despite being a really nasty Mad Scientist, Suchong's audio diaries typically have a twist on them that makes them darkly comedic. This includes his own, extremely satisfying demise.
Suchong: (On the Telekinesis Plasmid) "Cannot stop speeding bullet, but can catch and throw fast moving object. Problem not with Plasmid. Problem with reaction time (Beat) Suchong just get new idea for Plasmid."