Paper wallet
Laminate it
Keep in bank safety deposit box (~ $15 - $25 per year)
Tattoo private key microscopically camouflaged within another tattoo on your body.

Make sure trusted family member has access to bank safety deposit box in the event of your death. Also inform trusted family member of hidden tattoo for private key(s).

Another option could be to keep copy of private keys in separate safety deposit box at a different bank in case you don't like tattoos or in case you die in a horrible fire and your body is burned to a crisp or you get eaten alive by sharks or a pack of wild dogs.

Also be sure to give trusted family member access to this second safety deposit box in your death will.