Poloniex is probably one of the safer options, since they do use cold storage. the only problem is that if you need your btc out in a rush, it will take some time for poloniex to withdraw your coins.
yes no doubt that polonex is good option, but i think that most of the exchanges are safe and reliable for investment, i mostly use blockchain and yobit. they are easy to access you can deposit or wedthrawl your money in no time. although there are chances and not only for yobt but about every exchange to be hacked just like bifinex, therefore i most put a limited amount of bitcoin in exchanges and mostly use multiple accounts to distribute my bitcoins there.