No key no coins. The safest way to store your private key is to store it offline on a piece of paper. But what happens if someone finds that piece of paper with your seed?
You should first cut up your key into different pieces like horcruxes, so even if someone finds a piece they won't be able to
obtain your entire private key.
The safest way is to keep parts of your private key on you, but carrying around a piece of paper is impractical. The best way is to purchase a custom ring and engrave a piece of your private keys inside your ring.
For example. Say you have a 16 word seed, put 8 words in one place , 4 in a ring and 4 in a ring. Make sure you write have something that keeps track of the order, if you can't remember. Tungsten carbide rings are about $100 -$200 and are very durable. I recently purchased one on Etsy and I love it. It looks good and it also secures my coins.