Poloniex is probably the best In all terms (BTC/Altcoins). If we speak about their security then they use 2FA for our accounts and they also hold most of the funds in cold storage , which means even If they get hacked , the losses will be pretty small. website is user-friendly and the UX/UI looks perfect (have both Dark and light theme as well) , there is no need for any kind of verification (of documents) unless you want "really big limits" which most of people don't need usually. Moderators are always here in the Trollbox to help you In case you have an issue or a question too. As for the trading , It contains probably all Altcoins that you may want to buy/sell unlike other exchanges where there isn't much of them.
I agree with poloniex because there are big players or traders in it. Also, big volume is being traded in poloniex. C-cex is also a good one,just set an order and leave.Hassle free and less intervention. Yobit is fun also to trade with more players.Just be vigilant always with your trading.