Which one is the safest way to store bitcoins? I often hear that becomes thefts. I guess some of them are not heard even.
I think online wallet is still safe to use if you are using a special computer just to open an online wallet and is certainly to be equipped with other added security.
Online wallet is never safe as you are relying on third party to make your bitcoin transactions and to hold your bitcoins for you. They can be hacked any day, their database and your private keys gone even if you have your computer with top security. Second best are desktop wallets like Electrum in which private keys are stored on your PC and you are in charge. Ultimate security is a hardware wallet like Trezor or Ledger.
Stay clear of online wallets.
I can only agree, I see online wallets as a good thing only providing they support Trezor - so I can keep the private keys in my hands and use the web wallet as an interface to the blockchain. An example of a good web wallet is myTrezor.com or Copay.