Where it will not be compromised by 3rd party risk, or by your own doing. Preferably in your head, but also in your cold storage device(s), and in steel or titanium placed where you deem most safe. Give nostr:npub1lrj3lsskez59qu68v5fpnur7ythe7s5e4qcelclha6t6r9q5pqgsl2mzx2 a call if you want to ensure you do everything in your power while being guided by experts.