Which wallets store users private keys on the cloud?
Online wallet providers, like Coinbase (often referred to as bitcoin banks) just don't give users access to their private keys. The private keys to those addresses are kept safe by Coinbase, but as the old saying goes, "if you don't own your private keys, you don't own your bitcoins
As great of a company that Coinbase is, for introducing people to bitcoin and helping them make purchases, if they get hacked or an employee runs rouge, you could loose all your funds, it has happened to other companies before.
The same goes for leaving your funds on an exchange, if they shut down you have no way to recover your bitcoins.
Wallets such as the Bitcoin.com wallet
generate and store the private keys locally, on the users device itself.
If Bitcoin.com got hacked or our site and wallet servers went down, the funds in your Bitcoin.com wallet would still be safe. You would just need to restore your wallet using your backup phrase on another client to recover your bitcoins.
As long as you trust your own security, It is always advised to keep your bitcoins in a wallet, that you
control the private keys to.
Of course the best security is always hardware wallet, Trezor, Ledger, KeepKey
are the top ones out just now.
You can also use good old paper wallets, as long as you generate them correctly off-line, then your funds will be safe. Although safe from hackers, paper wallets can easily be accidentally
destroyed or lost. It is best advised that you laminate them, and keep them in a safe.