To ask a question about a specific or particular group of people or things, the interrogative determiner “which” is used: “Which is the poorest bank in the world?”
When “which” is used, the presumption is that the person being asked is familiar or has some prior knowledge about the character, nature, position, or standing of the group of people or things being asked about.
Other examples of “which” questions:
1. “Which cities in Europe have you toured?”
2. “Which math subjects are you taking this school year?”
3. “Which of the recommended routes are you taking?”
4. “Which of those guys would you rather date?”
On the other hand, to ask a general question about something other than people, the interrogative determiner “what” is used: “What is the poorest bank in the world?”
When “what” is used, there’s no implicit presumption that the person being asked has some familiarity about the subject or things being referred to. The question is open-ended and will permit spontaneous and unguided responses.
Other examples of “what” questions:
1. “What dishes have you learned to cook?”
2. “What unforeseen event has drastically altered the course of your life?”
3. “What made you think of changing jobs in mid-career?”
4. “What attributes are you looking for in a good teacher?”