42
According to the Hitchhiker's Guide, researchers taking the form of mice, which are actually 3-dimensional profiles of a pan-dimensional, hyper-intelligent race of beings, construct Deep Thought, the second greatest computer of all time and space, to calculate the answer to the Ultimate Question. After seven and a half million years of pondering the question, Deep Thought provides the answer: "forty-two".
"Forty-two!" yelled Loonquawl. "Is that all you've got to show for seven and a half million years' work?"
"I checked it very thoroughly," said the computer, "and that quite definitely is the answer. I think the problem, to be quite honest with you, is that you've never actually known what the question is."
Deep Thought informs the researchers that it will design a second and greater computer, incorporating living beings as part of its computational matrix, to tell them what the question is. That computer was called Earth and was so big that it was often mistaken for a planet. The question was lost five minutes before it was due to be produced, due to the Vogons' demolition of the Earth, supposedly to build a hyperspace bypass. (Later in the series, it is revealed that the Vogons had been hired to destroy the Earth by a consortium of philosophers and psychiatrists who feared for the loss of their jobs when the meaning of life became common knowledge.) Lacking a real question, the mice proposed to use "How many roads must a man walk down?" (the first line of Bob Dylan's famous civil rights song Blowin' In The Wind) as the question for talk shows, after considering and rejecting the question, "What's yellow and dangerous?"actually a riddle whose answer, not given by Adams, is "Shark-infested custard". However, this may also refer to the Vogon Constructor Fleet that demolished Earth, in that they were yellow and most certainly dangerous.
In one of the books, Marvin mentions that he can read the Question in Arthur's brainwaves. This does nothing to cheer him up.
At the end of the first radio series, the television series, and the book The Restaurant at the End of the Universe (the second book of the five-book 'trilogy'), Arthur Dent (as the last human to have left the Earth before its destruction, and therefore the portion of the computer matrix most likely to hold the question) attempts to discover the Question by extracting it from his unconscious mind, through pulling Scrabble letters at random out of a sack. The result is the sentence "WHAT DO YOU GET IF YOU MULTIPLY SIX BY NINE".
"Six by nine. Forty-two."
"That's it. That's all there is."
Since 6 × 9 = 54, this being the question would imply that the universe is bizarre and irrational; on the other hand, there is no proof that this was the actual question. After all, Arthur Dent composed only a minuscule fragment of the vast and complex computer matrix that was the Earth, and besides, it was stated that the computer's run had not finished when it was destroyed. In addition, Arthur and Ford realized that the original ape-like inhabitants of Earth were displaced by the Golgafrinchams, which could account for the irrational nature of the question in Arthur's mind (as he himself is a descendant of the Golgafrinchans). On discovering the question in the original radio series, Arthur Dent remarks: "I always said there was something fundamentally wrong with the universe."
Another thought as to the false equation in the Hitchhiker's Guide was that the program (Earth) would have run correctly if not for the crash landing of the Golgafrinchams. This race introduced error into the program and thus turned what would have been the equation 7 times 6 = 42 into 9 times 6 = 42.
It is also possible, given Adams' often bleak view of technology, that the 6 × 9 = 42 answer is meant to indicate that the Earth project was a flawed design to begin with, one that was always going to produce the wrong answer even if the program had been run successfully.
It was later pointed out that 6 × 9 = 42 if the calculations are performed in base 13, not base 10. Douglas Adams was not aware of this at the time, and has since been quoted as saying that "nobody writes jokes in base 13." and also "I may be a pretty sad person, but I don't make jokes in base 13."
Alternately, some have suggested that the question may be, "Pick a number, any number." Although this is not exactly a question, Marvin the Paranoid Android asks Zem the mattress in Life, the Universe, and Everything to pick any number.
"I gave a speech once," he said suddenly and apparently unconnectedly. "You may not instantly see why I bring the subject up, but that is because my mind works so phenomenally fast, and I am at a rough estimate thirty billion times more intelligent than you. Let me give you an example. Think of a number, any number."
"Er, five," said the mattress.
"Wrong," said Marvin. "You see?"
:footinmou 
Or maybe Foxy's Folly 
But PhotoShop causes UFO's.
