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.
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 apporach 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.
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.