Really, all of the game's dialogue would be on this page, but to be specific...
For some, the first spoken line in the game is one.
Isaac Wassup, motherfucker?
"You ever gonna tell anyone what that fuckin' 'G' stands for?" G's response is simply "No."
The door opens up to a staircase, and a severed head rolls down it...
Isaac: Shit, man! That ain't right!
When Isaac calls G "diaper-shit".
Isaac and G are about to ride off in Isaac's car, also known as his pride and joy. Said "pride and joy" promptlyblows up.
After The Screamer is defeated, Isaac and G notice that there is a phone ringing from inside of her. Isaac refuses to open her up, because he "manicured only yesterday" and isn't about to stain a freshly-manicured set of nails with it.
This exchange in "Carny", once G and Isaac meet their first "football" mutants:
The entire game is one big Crowning Moment of Funny, especially due to some of the weird phrases you have to type, or getting a word or phrase that's perfect for the situation (like a zombie jumping out of nowhere and forcing you to type "Boo!")
The Tower in Mission 3 gives the player questions, and then asks them to type the right answer. One of the possible answers to "Which of these is bigger than a bread box?" is "Yeast infection."
The question-and-answer format of the Tower? It's reused again for the final phase of The Emperor. This time, all three possible answers to each question are "correct" (however, they influence which ending you get). The answers range from semi-serious to outright silly. Some of the example questions are:
Goldman jumping off the building like in the original HotD 2, followed by an explosion and James's Stunned Silence.
Goldman bungee-jumping off the building, bouncing back up, and then burping.
Goldman letting it rip and then taking off into the sky, again with James's speechless reaction.
The credits sequence, which replaces the ending theme with one of the stage themes and shows a row of glass tubes. Typing the staff names out causes one of the zombies to break out of their tube and start dancing. By the end, the entire row is dancing.