I don't think that there is the safest way to store Bitcoin. No one will guarantee that one will be safest enough to save your BTC even in online or offline wallets. Because we know that besides the hardware or software that we are using, there will be also probabilities of human error, making mistakes of sending in wrong wallets, forgetting in saving the data to access the wallet, losing the wallets and others.
Moreover if this is about online wallets, there may be viruses, hackers, scammers, and also other probabilities of losing assets from the online wallets.

Are you sure about this?
Well, it may be very portable to bring our smartphones everywhere and we can check our assets every time as we like by using smartphones. But this is also risky because we don;t know whether the phone will be broken or lost someday, and the assets will be also lost.