The Sheldon Prime: 73

73 is the 21st prime. Its mirror, 37, is the 12th prime — and 12 is the mirror of 21. In binary, 73 is 1001001, a palindrome. And 7 × 3 = 21, the position of 73 in the primes. These interlocking properties, noted in the TV show The Big Bang Theory by Sheldon Cooper, make 73 uniquely self-referential. Mathematician Carl Pomerance has since confirmed these properties are genuinely unusual. Whether 73 is truly 'the best number' — as Sheldon claims — is left as an exercise for the reader.