I have 2...

1.  What have you looked at and/or made offers on? 

(I want to know what the list price was, what their offer was, why they have not bought yet or why they have not made any offers)

2. What do you consider a good deal?  (This is my favorite question, because too many buyers have unrealistic expectations, and now is the time to have a reality chat...not later, when they make a low offer and think it is your fault when it is not accepted)

I also want to know about the last house they bought or what they live in now. Actually, I want to know everything about them, so I ask a lot of questions.

Eve 

There are all sorts of questions I ask, but I want to know not only what they are looking for, but specifically what they need (which are often different)! And what their lifestyle is like. And of course are they pre-approved and for how much, and how much cash do they have.

I'm not sure I have a 'best' opening/open ended question, because I go by instinct/experience, but I always get clients to tell me about themselves first (the 5Ws), where from, what do you seek to do, plans, money, jobs, where do you want to be.... this sort of interview questions... I want to get a fix on who they are and I find a way for them to tell me basically why they are in my office and why we are meeting....

Buyers seem to naturally just open up to me and tell me more than I need to know. I am told I am easy to talk to.  Conversations flow easily.  I get what I need to know. 
The process always starts when a buyer gets pre-approved for their loan, ideally underwriter approval, and they provide me with proof of funds.  

What are you looking for in a home?

Asking a buyer how long they plan to be in the house they buy can lead them down the path of telling you about their job and personal life.  

For me, it's more of a connection I feel versus a particular question...it's nice when it happens but/and better to discover any issues sooner than later.

I just start talking to people and asking questions... I've been doing it so long I do it unconsciously, but always get the answers I want/need. What do I need to know? Have they been preapproved? Have they worked with another agent? Do they need to sell a home first? What kind of area or commute do they want? What schools are important? There's no end to questions.

"If we find you the perfect home today, are you prepared to make an offer?"

The two that will give you just about everything you need to hear answered (as they lead to further conversation, which is what I think you're looking for):

1. Tell me about your lifestyle - what do you (and your family if it applies) do on a daily/weekly basis?

2. Tell me about your current home & neighborhood. (If they are first time homebuyers, it's tell me where you see yourself living - community & home). 

Both of those questions will keep the conversation going for as long as you need to get all of your answers! As well, your new clients' affordability will often dictate very specific communities - at least in our area it does. 

Those below median price point are getting tougher and tougher in our low inventory market!

Where do you currently live? What about the place is prompting you to look for a new place? What are your needs? Anything else?

Thomas,

First thing we would do is establish whether we are a match, and if they have a sense of humor.  After that it flows.  A

I want to know where they currently live, and what the home is like, whether they like it or don't like it and why (and I might stalk their current home online if not local).
I want them to describe to me every possible thing that they would want if they could get everything that they wanted.
This will tell me their taste and their expectations and let me get inside their gray matter.
That will also tell me if their eyes are bigger than their pocketbook.
Many many of our home buyers are from Southern California. If I know what area they're from, I'll have a fairly good grasp of what they are used to.

My top 3 

Why do you want to buy

When do you want to buy

Where do you want to buy

Describe your dream home to me.

What are you looking for in your next home that you don't have in your current home?

Tell me about your dream home?

"What's important to you about moving to the Tampa Bay area?"

Identify the 'transformation' then find the right place where it can occur.

Parked and reading...

Thomas J. Nelson, Realtor, ePRO, CRS, RCS-D - that 'Magic Wand' question from Brian Buffini...

I ask them what their fear is.

  Hi, John and Mary.  You say you want to buy a house... tell me more about what you have in mind.

  What do I want to know? 

    I want to know that they are ready, willing, and able (especially "able", as in show me the money).   I want to know what they have already seen, whether touring houses, or surfing Zillow.  Have you met with any other Brokers?

What type of house are you looking for?

What do you need from me to make this successful?

What's your goal?

Ask for the name and phone number of their mortgage loan officer.

My first question is usually "Tell me a little about yourself?"

But when you ask this question, you had better shut your mouth and give them time to talk. Then listen.

My question would be, what do you know about the home buying experience? This would tell me just how long they have been looking, have they talked to a lender and if there is a parent advising them.

I have a long list of questions to ask.

I tell them I will show them 6 homes and after the second home I will have them rate them 1 to 10 and why something was a 6 and what would it take to make it a 8

Please...Tell me what it is that you are looking for.

I do not have  a script.