Depends on the purpose. For documents (especially those on which people work collaboratively over long periods of time) I find YYYY-MM-DD ideal. It spares the issues around day or month first when Europeans and people from the US work together, the document are easy to sort and, if it takes more than a year to complete the project (as is often the case in research) things don’t get messy.

The fourth kind is the reason I hardly ever pet any cat XD

I would love to see a massive boycott of MacDonald everywhere (at least in the US), I am surprised that this hasn’t come up somewhere online yet. That would be powerful signal IMO