Safety should be our priority in terms of our assets in cryptocurrency industry so it is also an investment. I think the best way to store Bitcoins for long term purposes is by keeping it out of the internet like paper wallets, hardware wallets or anything that can be weather proof.
This is the fact, but I will prefer to use open source because I can know the codes my wallet is using, I will prefer hardware wallet. There are lists of ole source hardware wallets in some topics.
Open Source Hardware wallets[]
I will prefer Trezor it is common and safe to use