You are over thinking it. The answer should always be the school you are interviewing with, followed by some well reasoned concept as to why. If they ask how many schools you are applying to, I'd say "well to mitigate risk, given the nature of MBA admissions, I'm applying to X, but your school is my unequivocal top choice.."
One thing that I'd add in here is to have a corss section of things that have drawn you to the school you are interviewing at. Recruiters, fellow students (whom you have hopefully met some), professors, unique offerings at the school, clubs, etc. Just like in your essays, you should show depth.