Ooh, I have a few different favorite numbers, and all for very nerdy reasons. One is 1729, because it is the smallest number that can be written as the sum of two cubes in two separate ways (1729 = 123 + 13 = 93 + 103). Another is 842 just because I like the way it halves the value of each digit. And finally I like 73 because it's the 21st prime, 37 is the 12th prime, and 7*3 = 21, which I find to be a wonderful set of coincidences. Oddly enough I don't have a favorite single digit number.