Charon, the ferryman of...the sewers (aqueducts!), who is not terribly fond of the beautification project Neverwinter's undergoing, as the finer points seem to be giving things pretty names...
Charon: "Sewers become 'aqueducts', waste becomes 'surplus resources', and my cushy job at the palace becomes tour guide to the FREAKIN' SEWERS, BUT I'M NOT BITTER!!!
Later, when you ask if he can take you downstream:
Charon: "Well, I can't leave this spot. An emergency could arise, and my specific expertise could be required. IT COULD HAPPEN!"
A (plot irrelevant) note found in the Host Tower of Luskan: "Please inform the apprentices that they are no longer to dump their failed alchemical experiments in the sewer." One can only wonder how these potions affect the water under the city. Considering how during your trip through the sewers, the place is full of ghouls, the water is all an unhealthy dark green, there's a general greenish pall, and an enterprising gnome has already found a way to channel the special sewer water into magical potions, the message is a bit late.
In general, any mad ramblings within the game (usually found within prisons).
In Shadows of Undrentide you can find the journal of a dragon that controls a tribe of kobolds. The entries are the Surrounded by Idiots commentary you'd expect.
"Must look into creating non-freezing ink. Damn kobolds tried thawing the last bottle over a fire and it exploded. Upper cave still dotted with ink marks." "Swam to the outside today and encountered a group of orcs on my favorite ledge. Why do orcs have to taste so bad? Note to self: raid for salt." "DAMNATIONS!! Why must every greedy plan of mine end in ruin?!"
In Shadows you can visit a temple where worshipers are meditating on Ao. You can take a test where you must answer a riddle. Your answer is apparently so profound the worshipers assume you are Ao, and start begging you for wisdom and guidance and give you gifts as shows of loyalty to you. You have the option of going along with it or telling them to knock it off.
In Shadows you can find a Shield Guardian golem fighting enemies. Defeating them and talking to it invites one of the most hilarious examples of Insane Troll Logic ever.
"You are an intruder. You destroyed other intruders. Destroy of intruders is a job for guardians. You destroyed intruders, therefore you are a guardian. How may I assist you, guardian?"
This is followed by you pointing out the guardian isn't very good at his job of protecting the master by telling it the master's skeleton is behind it. The guardian insists that the master is simply in quiet meditation and is so wise and powerful he does not need flesh to live.
Shadows of Undrentide and Hordes of the Underdark's funny moments usually deal with Deekin's overeager dialogues.
Deekin and Nathyrra discussing what to put in a cookbook for dragons.
"Drows be fatty. Gots it."
If you're travelling with Deekin and Sharwyn, Deekin will ask her how come her red hair changes color near the scalp. Sharwyn pays him off to never bring it up again.
Heck, all of the inter-party dialogues between Deekin and another henchman are pretty damn hilarious.
Nathyrra's random ramblings about the state of her pack.
If Evil Aribeth is in your party and is the Sleeping Man's True Love, and the Sleeping Man is awake then some pretty hilarious dialogue ensues.
And the developers were probably aware of how annoying Deekin could be when he stops the player yet again in Chapter 2 of HOTU to ask for a bathroom break.
When you confront the Final Boss in Hordes, this exchange takes place:
Mephistopheles: Last I knew, I thought I had trapped you for all eternity in an icy little place called Cania.
Mephistopheles: A...a chambermaid, Master/Mistress? But I was meant for greater things! You would reduce me to turning down bed sheets and emptying the chamberpots of drunken mortals? ...It shall be as you command...