What is the highest number in a sudoku puzzle?

The puzzle known as "Al Escargot" (the snail) is currently considered the hardest Sudoku puzzle. It was created by a Finnish mathematician called Arto Inkala.

One of the hardest Sudoku books available is "Extreme Sudoku" by Antoine Alary, not to be confused with "X-TREME Sudoku" by Nikoli & Co. or "Sudoku Xtreme" by Christopher Monckton, which are both an order of magnitude easier.