I'm a little like Mystyfyr: mine is e because in my education, the damn thing shows up all over, when you least expect it, and does such unique things as well. And of course because it's a key player in: e^(pi*i) +1 =0, the most elegant formula in the known universe.
It used to be 5, because I liked the way the digit looks and is pronounced. And it just seems like a nice, convenient number. When I didn't really know anything about odd and even numbers when I was really young, I used to always think with great conviction that five was even, because it "looked like an even number". I associated "even" with being devious. I couldn't comprehend that it was "odd", because I interpreted "odd" numbers to be weird, and I didn't find 5 to be weird at all.
But now it's fourteen, because halved it is 7, and I like 7. There's some history behind that... Doubled, it's twenty-eight, and I like that number for many reasons. It is my mother's favourite number, it is the age of one of my obsessions, it is an age mentioned in a song which I became obsessed with for several months. Plus it seems like a nice age too. Fourteen in particular? I love the way it looks written out as a word, in chapter fourteen in a book I have, my favourite character is depicted, 1+4=5, 1&4=14, among some other reasons. Also, I met my first boyfriend when he was fourteen years old, and I always think that his fourteen-year-old-self was an absolute state of perfection.
Also, this guy I was once obsessed with, his birthday is on the 4th of October, the 4/10, 4+10=14. And my first best friend's date of birth + my first crush's month of birth = FOURTEEN.
It seems like I've thought this through quite a lot.
Ironically though, I absolutely adore Chinese culture (I want to be a sinologist) and I just found out today that they consider fourteen to be a highly unlucky number