I like an exchanges that easily use, where you can easily locate trade on it without any form of difficulties. Also an exchanges that has low transaction fees with fast interexchanges transactions. I also like exchanges that has good security features and offer some sort of incentives like voting of a coins to get listed freely and an exchanges that has good staff supports and efficient customer services.
All these make up a good cryptocurrency exchanges.
Well, UI may be something that could be of an advantage, but that does not really matter since you can use your API on any exchange to trade on any other preferable platform anyway. For instance I trade on bittrex, binance and Kucoin using tab trader on my android device.
However, what attracts me to an exchange most of the time is their reliability in terms of support, security and how reputable they are. However, fees is also one thing to consider when being attracted to an exchange which is why my best so far is binance, followed by Kucoin.