Referring to someone as an idiot or their behavior as idiotic seems to be a favorite thing for many DS characters (Roger and Jason and Jeb, just to name a few off of the top of my head) with insulting, self-effacing or other intentions. But I think my favorite time is in 1795 in Ep #387 when Andre calls Trask "a complete idiot!" It's what most in the audience are all too probably thinking themselves - and the look of self-righteous indignation on Trask's face afterward is priceless!
I wonder if the 2nd most popular practice was calling someone a fool or foolish. Barnabas, Quentin, Angelique, Burke, Jeb, Bruno (possibly the biggest fool of all), and especially Joan Bennett's and Louis Edmond's characters got in on it. "You fool!" loses its impact, I think, because it gets tossed around so often. But one incident sticks in my mind was Barnabas calling Julia "a bumbling fool," ouch!