Very debatable. I've always found YYYYMMDD to be the most pertinent format as it gives you the "most important information first". As in, DD is unusable if you don't know the month, and the month is unusable if you don't know the year.
I like to picture a world where everyone use YYYYMMDD, the Metric system and English for technical terms (I'm from France and I hate when we invent new terms for technical terms).
When dealing with data processing, the concerns are obviously different. But then it barely matters which order you use anyway.
You may know what month something happened to you but you may not know the day in which case you would just write 201308 which is a perfectly valid date. You may know what day and what hour someone was born but you may not know the minutes or seconds (ex. 2013080506).
By using it, there is no ambiguity because the year signals the order of values. It also sorts better than any of the other methods.
For most "real life" cases I could find it was smarter to put YYYY first. And for an impressive number of cases, the day of the month didn't really matter and could then be omitted more easily.
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.
You get the 2nd shortest duration first (tens of days) then the shortest (days) then the fourth shortest (tens of months) then the third shortest (months) etc
With the ISO standard every character symbolises a larger duration than the one that follows, so to get a strictly increasing order you have to reverse the string entirely - today would be 92-11-3102