Which is the best media for long term storage?

[Xanxi]

Hard-drive stored in a safe place, if you want your datas at home, or space storage somewhere on internet if you want it outside.
Anyway, double save your datas

[Xanxi]

A stone.

This is a pretty robust medium and can last many thousands of years.

Unfortunately the capacity is rather limited, even if it was enough for the 10 commandments

I fully agree