Yeah, speaking of the type of device with the highest security, airgapped devices should be the safest and they can be used as primary asset storage. Since you prefer mobile, I found open source software that allows your unused android/iphone to become an offline wallet. Read the review here if you're interested https://bitcointalk.org/index.php?topic=5342004.0.
Or if you have a lot of bitcoin (say 1BTC or more), you should be able to afford a dedicated hardware wallet. https://thebitcoinhole.com/hardware-wallets