to make things as not confusing as possible, my rule of thumb is:
yyyy-mm-dd (yyyy instead of yy ensures that it’s not mistaken for dd-mm-yy) (hyphens can be replaced with underscores)
dd.mm.yyyy (yyyy same as above) (really dislike using for filenames, sorting doesn’t work)
mm/dd/yyyy (only if there is no other choice)
edit: mm/dd/yyyy vs mm/dd/yy doesn’t matter because both make 0 sense already
edit2: i forgor to say that yyyy also avoids y2.1k and subsequent issues
to make things as not confusing as possible, my rule of thumb is:
The first one you listed is an ISO standard date format, and is the only way to go :)
if i write a date on paper i tend to go with 2, but yes
I dunno. If the date is between 2001 and 2012, I prefer YY/DD/MM. So August 4th, 2005 would be 05/04/08.