Numerology was never really than interesting. 616 OR 666. Mystical systems bother with numbers too much.
I would probably say the Pagans or who ever founded Christianity chose the number 666 for a specific reason.
There have been so many people digging into 666 over the past 1000 years. 6 Being the number of "man" and 7 being the number of "god". 777 is the holy trinity of perfection, 666 would be the trinity of man, being uncomplete.
Supposedly in the occult 6 is meant to represent man, and a number repeated 3 times is considered to be in it's Ultimate (or Complete?) form.
So the number "666" would mean: The Ultimate Man.
I like to avoid using "666" because I believe it has significance to people other than Satanists.