If you can ask someone only one question that you believe will tell you all you need to know about someone, what would be?
Mine: What do you put on your hot dog?
And if you answer “ketchup” then not only do you make the Baby Jesus cry (I had to say it) but you will never get my kidney either!
I don’t think there is just one question I could ask to find out everything I need to know about a person, but “Do you like The Simpsons?” is a step in the right direction. As long as they reply with an enthusiastic yes.
I think that it’s very close to what you just asked. I’d ask “If you could ask one person in history one question, who and what would it be?” I think that that is very revealing.
What one person do you most admire?
One question I ask people is “If you could go anywhere you wanted on vacation, where would you go?” Another I ask is “What is your idea of the perfect job?” Both questions tell me what the person really loves.
To answer Tretiak, mustard and chopped onions.
Munch, what does it tell you when you ask that question of someone who doesn’t have a snappy answer prepared? It’s not the kind of thing one walks around knowing about oneself. Puts me in mind of the kind of questions job interviewers ask. What exactly are you going for, there?
I first read your post about ten minutes ago and I haven’t come up with an answer yet.
Now, Jwalk’s question about the Simpsons…if someone answers “yes” to that, I know we’ll always have something to talk about.
What kind of music do you like/listen to?
“What do you read for fun or pleasure?” usually tells me volumes about a person and whether or not we will have enough in common to find out more.
I’m totally stealing this from This American Life, but it’s generated several very interesting conversations:
“If you could have one superpower, and you were the only person in the world with a superpower, would you rather have the ability to turn invisible, or the ability to fly? Explain.”
I’m always leery of questions like this, though, because my experience is that the person who asks it generally has their own clever answer, and instead of listening carefully to your response they’re just waiting for their turn. Not saying that’s the motivation of anyone in this thread, though. . . just sayin’, is all.
My favorite icebreaker, though, is “What are you reading?” Helps if they actually have a book or a magazine, but if they look at you blankly and say, “Uh, nothing,” well, that tells you something, too.
And I can’t think of a question for a historical figure, either. 
I’d be afraid they’d say, “Geez, I’ve answered that a hundred times. . . Don’t you know how to use Google?” 
As a person who makes his living asking questions, my question when I am in one question situations is very similar to yours atypicalcarl.
Mine is, “Who’s your hero?”
Generally speaking, I can learn more from that one question than five or six others. I actually prefer mine to yours though. I find that the shorter the question, the quicker the answer. I also find that quick answers tend to be the most frank.
TV
Try this…
What do people most often get wrong about you?
And if I tell you that I like corn dogs better, with mustard, what does that tell you about me? Just curious.
What do I need to know about you?
When finding out about a man my very first
IMPORTANT question would be… ( if you KNOW he has one living )…
Do you love your mother?
I find it best to ask the question in the middle of a totally unrelated conversation, just out of the blue.
The reactions and the answers speak v o l u m e s.
I’d ask them to see their CD collection. You can tell a lot about a person by what kind of music they listen to. By the way, I actually like ketchup on my hot dog.
runs and hides
What’s your SDMB name?
Here is the question I would ask:
“If you can ask someone only one question that you believe will tell you all you need to know about someone, what would it be?”
I’m serious. This exact question was posed as a solution to the Turing Test.
How would you define ‘love’?
Take 'em to Disneyland, then ask them, “What do you want to see first?”
When my then-girlfriend answered “Mickey Mouse!”, I knew I had to marry her.
You can learn a lot, but not everything, from any one of the questions below: