There is no best. I've just finished 'Best Detective Stories of Cyril Hare' which was brilliant (in my opinion). Short stories with an excellent twist. Ask me in 12 months time and I'll probably give you another answer.
2) What is the worst book you've read?
If it was that bad I'm unlikely to have finished it. And probably won't remember what it was unless I hear it recommended and then I'll know I didn't like it.
3) What is a favourite book from childhood?
A A Milne's Winnie-the-Pooh stories, because I continue to enjoy writing the characters.
4) What is a favourite book from adulthood?
Again, I doubt there is a favourite. My best answer at the moment (no doubt that will change) would be E C R Lorac's books.
5) What is a book you've read a number of times?
Arthur Conan Doyle's Sherlock Holmes stories. (Don't look surprised!)