51

"This appears to be the first uninteresting number, which of course makes it an especially interesting number, because it is the smalles number to have the property of being uninteresting. It is therefore also the first number to be simultaneously interesting and uninteresting." -- David Wells: The Penguin Dictionary Of Curious And Interesting Numbers, 2nd ed., p. 107.

Being revised in 1997 this seems to be outdated information: in Robert Munafo's Notable Properties of Specific Numbers 51 is categorized as a Motzkin number.