If God had a name, what would it be
And would you call it to his face
If you were faced with him in all his glory

What would you ask if you had just one question

Which is a good point. What would you ask God if you had one question? Luckily, by definition, the obvious question has already been answered; Is there a god? So we can move onto juicier stuff.
If we want to get maximum value out of the one question, we’ve got to think of the benefit to humanity. Two things come to mind, depending on whether we want to be forward thinking or backward thinking. Both are requests for technology that would help everybody.
“What is the way to travel faster than the speed of light?” With that we would, presumably, be able to conquer the galaxy and spread humanity out. Let’s face it, all our eggs are in the Earth basket right now. Of course God could be a jerk and reveal a method that has too high a cost to use. (First you need a planet made of antimatter…) If we have time we could probably get some scientists and lawyers to come up with the ideal version of the question. Also, it takes away humanity’s victory in finding the discovery. It wouldn’t feel like we made it and deserved it, but had it handed to us. Stuff we get for free isn’t as appreciated.
The other technology, which I can’t phrase in the form of a question here, is a lesser known science fiction trope. Basically, a machine that allows people to see anything that happened in the past. With that we could answer all the questions of history, but more importantly, always have a credible witness to any crime. Everyone would have to take responsibility for their actions.
Of course it is also possible to massively abuse the technology if it was common and not just used in the hands of licensed professionals. It could destroy all privacy when anyone could see what you were up to at such-and-such a time. If used well, I think this would give a good way to clean up humanity. But after looking at what I wrote, it also looks like a good way to start a despotic government.
So I guess the first question is my top choice.