I would not trust an online storage site or an exchange with a large amount of money. I would only put money on an exchange for trading and I would keep some stored separately in a secure location. So the question is what is a secure location? Most people would say an offline paper wallet is the most secure. If you know that no one has the private key, then that probably is the most secure. You would need to make sure you keep the wallet address and private key where you can get to them but they are secure like in a safe deposit box or a safe.  As long as you do that, you should be fine.

Then the problem is what happens if you die and you want a beneficiary to have access to the coins. For this situation, you may need to leave instructions on what to do with the keys in a will or in the safe. I have heard of people keeping the public and private keys on a thumb drive, but I have heard that the data may not last forever on those, so a paper wallet is probably best.