Fermi Questions are pretty useful if you need to gain an understanding of a candidates ability to process their own thoughts in an uncomfortable situation. http://mathforum.org/workshops/sum96/interdisc/classicfermi.... It depends on how you define 'best'. Do you mean the most humorous, most obscure or are you referring to non-tech questions that reveal a lot about your candidate?
The latter-- non-tech questions that reveal a lot about your candidate.