I assume it's probably been said somewhere before, but The Ultimate Question in Hitchiker's Guide To The Galaxy is probably "Pick a number, any number". Two characters in the show know the question, and they both ask it.
Eddie asks the crew to "Pick a number, any number", immediately after they finish asking about the Ultimate Question. They treat it as a request to generate an improbability number to make the ship go, but it could be Eddie trying to help. It's been established that he knows the Ultimate Question.
The other possessor of the Ultimate Question is Marvin, who scanned Arthur Dent to find it out. Later, to impress a mattress, he asks the mattress to "pick a number, any number", as evidence of his intellect. The mattress' guess of "Seven" is wrong, so it must be "Fourty-Two".