[Zekio]

YYYY/MM/DD is the best format(edit: it's the same one used in article ofc), since it is also sort-able

[MJSplot_author]

I like YYYY-MM-DD as it is also sortable and doesn't require any escaping as a file name on windows or Linux. It's part of ISO8601.

[Zekio]

slashes or dashes makes no difference to me, still the most readable formats, since we are used to the biggest number first, from our numbering system :)