2007.12.24 is better for computers (sorting and such), well technically a UNIX epoch timestamp would be best, but what ya gonna go?
But the European format of 24/12/2007 is best for people. Think about it; the day changes the most, then month, then year. By having the date (which is different most frequently) right at the beginning, you can look at the date and straight away know what day it is. I could guess that it was December 2007 without looking; it's been the same for 24 days already!