What one question tells you the most about someone?

'How have you been?' This question tells the most about someone according to me.

Reason??

A person will respond with some very limited number of answers and by reading them you can get a deeper and psychological meaning of something they are trying to say. 

• 'I have been fine...thank you'- States slight awkwardness in expressing the current situation. You can dive deeper to know how they are feeling. Being 'fine' is a thin line between okay and not okay. 
• ' I have not been okay lately, but thank you for asking'-  They gave you a direct link to connect to their feelings. Ask about their thoughts and what is not making them okay later. 
• 'Things are well/okay'- Clearly states that something must have been wrong in their life and now they are beginning to cope from it. 
• 'I am absolutely delightful'- It could explain a sense of euphoria or happiness inside the person you are talking to. Try to keep in touch and take time in getting to know them; if something in their life goes wrong, be kind enough to ask them. 

These are some of the few answers we may get after asking the above mentioned question. By this approach, we may get to to know most of the things about someone.