3: 3 is a very nice number, mostly because it's short. It's also prime and can be used to make a very big number (TREE(3)).
7: Probably my favourite single digit number. Very nice looking, and can be written as the sum of two primes, and is prime.
37: Out of 100, this is statistically the least random number so it was bound to appear here. That's very cool but it is kind of overrated.