In my personal opinion, the most reliable and safest way to store crypto (that too for investment purpose) is by having a hardware wallet. There are various reasons to support this:
1. Hardware wallets are basically external devices that are only connected when a transaction has to be performed. When disconnected, they are isolated from PC and thus from the internet itself. This helps in securing your private key that’s stored inside the wallet.
2. In online wallets, your private key is stored on an online server by a third-party application (the software) and chances of your wallet getting hacked is very high.
3. The pin or password helps in securing your wallet, so, even if somehow, it gets lost, or stolen, then you don’t need to worry about your cryptos being stolen. But, you do need to remember the password as without it you won’t be able to access your cryptos.
With that being said, some of the top hardware wallets are Ledger Nano S, Trezor and KeepKey, etc. These wallets also provide support for numerous Altcoins apart from Bitcoin (Ledger having the most support, having support for 1100+ coins and tokens).