"Which floor" or "what floor"
For example if you're in the elevator, there a person coming in and you would like to help him/her to press the level button.
How do you ask?
"Hello, which floor are you going?"
"Hello, what floor are you going?"
"Which" is more correct, but "what" is commonly used.
I would say "Which floor are you going to?" if forced to use a whole sentence.
Simply "Which floor?" would normally suffice.