There are 20 possible answers to this, but I believe the one most likely to survive regions, regional settings and database settings is:
'YYYY-MM-DD'
...That's what I always use.
If you're talking about what column datatype you need, then I would recommend DateTime, unless you have some extremely unusual time capture needs. DateTime will store the time element - but it's always better to capture that up front and strip it out on queries if needed, rather than using Date and then wishing you had Time.