The single most important question for a Project Closure / Lessons Learned meeting is "Can / want / have to people speak honestly?"
You must ask it yourself before the meeting. If the answer is "Yes", other questions are not important because people will say everything what they have to say. If the answer is "No", I would google for "lessons learned questions" and take the first appropriate result. I also would ask minimal necessary amount of questions to reduce formalities.
By the way the only one question a project manager must ask on a closure meeting would be: "Is there something I must know but do not already know?" but this is the pure theory.
Да, по-моему "the single most important" - это не правильно, но кажется я его уже где-то видел.