First in your brain, maybe with anothet highly trusted human, then backed up in a bank vault in a completely different location, like other side of country, in the rare chance you die/lose memory
Discussion
Let's put all my keys to my self-sovereign/ p2p/ incensurable/ freedom/ decentralized/ be your own bank/ don't trust verify money in a centralize bank vault.
Don't do that, read the answer about keeping a decryption key there
On its own any banker doesn't know what it opens or the location of the encrypted material