A single question to learn so much about the other person?

Liberal or conservative? It's a dealbreaker for me.

It seems to me the one in your initial post is proving effective, at least for revealing some of how people think . Otherwise, maybe something about values (eg. What are some things you value most?
(Their why will likely follow, but in case not, some follow up could ask how they came to develop that particular value, or what experiences may have helped shape it).
Happy asking and learning!

Why are you the way you are?

What do you know with certainty?

If you are trying to find out the most from someone about a subject or quality that YOU care about, you must ask a specific question about what you care about. If you make that question open ended, you will get the most information but not necessarily the kind of answer you were expecting.

So for example if you care about chickens the most, and.a considerable amount of your life revolves around chickens, you might ask them what about chickens appeals to them and why.
A closed question for instance is
What kind of chicken is your favorite?

On the other hand what you care about most may be complex in a social or spiritual aspect.

if you care most about how someone is going to treat you, you need to dig deeper into yourself until you start identifying exact kinds of traits you find most important: someone who isn't going to put you between the bear and them, someone who will walk out with you from a situation where you are compromised in some way, someone who isn't the cheating kind, someone who isn't manipulative... The list goes on. A question in this area would need some self soul searching, but once you've found that, I think spending time with someone and seeing how they behave particularly with others in a subordinate position will give you a good idea about their character and whether it's a good complement to yours.
No easy questions in this regard!

Besides family... if there was a fire in your house, and you only had time to grab 5 things, what are they?

This can give a peek into what physical items are important to them. Some might grab a musical instrument, some might grab a photo album. Could begin conversations on why those things are important to you.

What makes you comfortable? I think this is good to know. Do they like people or no people? Do they like to snuggle with animals? Do they like sweatpants? Do they like it hot or cold? It could go lots of different ways.

or a similar one...

If you just had a great evening... what did it look like? What do they enjoy doing with their time? Watch tv? read a book? Go out dancing? Work on a hobby? Go to the bar/club? Play board games with friends? Play with their kids?

"Do you prefer Star Trek or Star Wars, and why?"

If I were given a one-way ticket to colonize Mars, would you come with me?

(this is more of a dating question methinks)

“If you had unlimited money or resources, what would you spend your life doing?”

I have asked every new potential date what their top five values are, and how those inform how they show up in the world.

It takes that one single question to weed a person out- or determine if they’re worth going out with.

It took over two years of trying but I have met my forever, and you better believe that question was asked :)

Joe Sharp wrote:Liberal or conservative? It's a dealbreaker for me.

Identifying as either of those is almost a deal breaker for me too. But I'd be more interested if it was phrased something like: "Do you think you lean more liberal or conservative, what does that mean to you, and why do you?" But I think this is pretty specific to the US.