I can't say I have any favorites, but from limited choices, the most interesting one is 0 (zero), given than infinity (∞) is not available in this poll.
9 because is cool ...lol Any simple multiplication of 9 = 9 e.g. 3 * 9 = 27 and 2 + 7 = 9 9 * 9 = 81 and 1 + 8 = 9