The Findus commercial taping. Just listen to Welles arguing over the poor grammar in his copy, offering to go down on the ad's director, and then storming out of the studio and remind yourself that this is coming from the same man who made Citizen Kane, because there is no doubt whatsoever that he was thinking exactly the same thing during that recording session.
"Now, what is it that you want? In your... depths of your ignorance, what is it you want?"
YMMV on this one, but if you're not a fan of Woody Allen, then you'll find Orson's dislike of the guy to be pretty amusing.