What single line of dialogue never fails to make you laugh?
There's a lot of them, but highlighting just one, it would be: "My wife left me and I'm completely alone" after failing the logic check with Titus.
Edit: Reading the replies has been hilarious. Here's another one I love: "I'm Dumb." When failing the visual calculus check at the building sign covered in bullet holes, followed by the morale damage, which on my first playthrough also caused a game over.
Somebody quoted one from the honor cop dialogue but I wanted to add "I'm scratching my ass, what's the big deal?" after Kim's reaction is priceless.
Edit2: Here's another, "The door was giving me shit" when answering why you broke down the door in the back after the shopkeeper gave you the key to it.
Edit3: When you kick the mailbox Kim: "Very cool" Harry: "Do you really think it was cool" Kim: "No"
Also, the entire failed Shivers check with the dicemaker