If you have high enough Persuasion, you can get Gar, the 'World's Smartest Orc' to start ranting about tea, in an entirely lucid and civilized manner.
Meeting Nasrudin is hilarious if you've got Virgil in your party, and quite funny even without him. Even the generally composed Living One loses coolness when presented with the incredulous revelation:
Player: WHAT?! YOU'RE NASRUDIN?!
If you talk to the Doc with low Int, he asks you your name. You answer "I am me."
If you have Intellect below 5 then your character's dialogue and journal entries are all changed to reflect their mental deficiency. This leads to many funny conversations when people try to communicate with your character. Some however approach it with snark, like the professor of demonology in the Tarant University:
Player: No, I mean it! I really stupid!
Professsor: (lowers his glasses and looks at you) Low hung brow, dazed look on your face... It appears that you are correct, my friend. You are indeed a complete imbecile.
Low Int conversations with Virgil are some of the funniest, not in the least thanks to the voicing. Not only your character keeps mispronouncing Virgil's name, he (she) can't even stick to the same name between one conversation and another. Virgil's chagrin is as understandable as it is hilarious.
If you play as a half-orc or another character with low reputation, you can ask an exceptionally friendly priestess why she's nice to you.
Lukan the Witless and his attempts at Sesquipedalian Loquaciousness, especially if you have enough Persuasion to talk your way past him. Or if you're brave enough: trigger a fight.
Player: (Paraphrased)"Infericous? It's inferior, you bloody pinhead!
Simeon Tor, the head of the Tulla mage school, really seems to enjoy jerking the player around, being needlessly cryptic about the proper way to contract the long-dead founder of Tulla, Pelojian. His astonishment at being confronted by an imbecilic Living One is hilarious (particularly if you've talked to Simeon as an intelligent character in the past). The voice acting as he interrupts his prepared lecture really sells it:
Simeon: Pelojia... wait, you are an idiot. How can this be? How can you be an idiot?