In my opinion, offline storage is the most safe, I recommend to invest in cold wallet
imo cold wallet are the best and safest option. i don't know you heard about Bitmarket which suddenly closed down, users lost all money because they had held their money on exchange...
You have only yourselves to blame if you store the coins in exchange wallets and then lose them. More than 80% of the crypto-exchanges have been hacked at some point of time, and this includes all the major ones such as BTC-e/Wex, Mt Gox and Binance. The only scenario in which I would consider storing some of my coins in an exchange wallet is if the desktop wallet for that particular altcoin is very heavy and if the net worth is lower than $100.