I'd say the most secure option is yet to be released, and it will be a hardware device that uses NIP-46 and software such as this: https://github.com/blackcoffeexbt/hardware-nostr-connect-device

In the meantime you could look at Amber. When the fedi app supports nsec import, that would also be a good option for mobile (you can use it securely right now with a fedi-provided key)