According to the internet search engine, Ask Jeeves, the top ten unanswerable questions, based on around 1.1 billion queries, are as follows:-
(If like me you had no idea why number 9 appeared in the list, seemingly it relates to the final episode of a popular US drama, The Sopranos. and lots of people have asked about what happened to the character "Tony Soprano". Sorry, I haven't a clue.)