Which is better - black, gray or white box testing?

It is a tricky test interview question. Most likely the interview tries to separate the wheat from the tares. Most likely, if interviewee is inexperienced tester, s/he will try to emphasize the gray box testing as silver bullet to slay all issues with application under test. The correct answer on this tricky QA interview question would be the following: it's impossible to declare one of the testing approaches to be better then another. It depends of Quality Assurance Engineer skill set, the type of the project, what is trying to be achieved during testing.