255

This number is a composite.

255 is the least odd integer greater than 3 which is not the sum of two distinct powers of two and a prime.

255 (3*5*17) is a sphenic number, i.e., the product of three distinct primes. The next sphenic number is 258 (2*3*43). Curiously, 3² + 5³ + 17⁴³ is prime. [Honaker]

The smallest number n such that 2^99+n is prime. [Loungrides]