Many people pick 7 because it's universally lucky; perhaps they want universal acceptance. Only the daring would pick 13 (or those asking for trouble). Picking 0 or 1 could indicate you're a loner. Three is a magic number and 4 is cosmic (if you want to know why 4 is cosmic, ask me about it some time). And some wise guy might choose pi, which is truly a transcendent number.
Mine is 74.
It has been at least since 4th grade when I painted the number in watercolors. I represented it in blue and violet because my synesthesia views it that way, and I've always enjoyed the superfluous cross the seven often gets.
I think, though, that letters may have an even stronger relationship to our personalities. Letters are associated with far more things in our lives, since every named thing and person contains letters. What are your favorite letters and what do you think they say about you? How about least favorite letters?