Perhaps this one, since there is no correct way to answer it. See I do not know the most difficult question to answer, because I have never been asked it. Now lets say I had been. Well even then it would simply have been the most difficult question for me to answer.

This does not mean that it would be the most difficult question for anybody to answer. To one person perhaps they had something extremely traumatic happen to them and the most difficult question is simply who, what, when, why, or how. Yet to somebody else that has had no such experience then the most difficult question could simply be are they willing to commit. Or perhaps what is πR².

The most difficult question to answer is either one that your mind simply has no reference or association to or one that your mind wishes to protect you from.