Usage of what and which

Question

What car should I buy or which car should I buy, what's the most correct one

Answer 1

When you're toying with the idea of buying a car but don't see a hundred cars lined up in front of you at that very moment, you ask:

What car should I buy?

When you're at a car dealership staring at a bunch of cars

OR

browsing through a catalog, you go:

Which car should I buy?

"Which" means you're assessing a finite number of very concrete cars. "What" means you're just speculating in general.