I like ISO format because then alphabetical order corresponds to chronological order.
Y-M-D and D-M-Y both have their advantages. Unfortunately my compatriots have standardized on M-D-Y, a convention that has no advantages that I can think of.
I've always found M-D-Y apt for many use-cases, because I often know exactly which year is being referred to - I just need to know the month and day. With M-D-Y, that information is immediately presented. It seems like a decent compromise.
My favorite format is YYYY-MMM-DD. That is, year, 3-letter (english) month, day (i.e. 2013-Nov-29). Only works for people that know the english names for dates, but eliminates ambiguity about which is the day and which is the month.