There's some good ones in this thread.
My dad's... "roommate" is probably the stupidest person I have ever met. I use "roommate" in quotes because my dad "doesn't charge her rent in cash" and never outright says anything deeper about that. *rolls eyes*
Her latest crowning moment was when she asked him "Who's Adolf Hitler? I've heard the name before but that's it." My outraged dad then spent the next hour going on a shpiel about the history of World War II and believe it or not, it was all NEWS to her. She had NEVER HEARD OF IT before, ANY of it. When my dad told me about this, I didn't even believe him at first.
Another crowning moment of hers was when she was going to get groceries at Safeway, and so she calls Dad as she gets there and asks if he wants anything. My dad, who is a complete jokester, replies "Yes, I'd like some powdered toast and some quail eggs". You and I can both imagine that 99% of human beings would laugh and realize he's joking. But no! The roommate then spends the next half hour wandering around the Safeway looking for those items. Dad finally gets a call from her later and wonders why she's taking so unusually long; she explains that she's still looking for the quail eggs and the powdered toast and can't find them. This outrages my dad and he asks "You mean you had no clue that I was joking!? What is WRONG with you!?"
I really don't have any clue how to describe their relationship. On one hand I honest-to-god believe that simple Darwin Ex Machina should have killed the poor lady off years ago, on the other hand I feel like she's the only creature in the world that can counter my dad's super-logical, super-cynical life attitude. The two of them crack me up so hard. Even though dad brings this pain on himself, yeah, that roommate is the stupidest human being I have ever met.
At least she's nice though.