Here is a consolidated list of best cryptocurrency exchanges with my comments:
Binance Offers mobile app and probably the fastest growing exchange. If you need to pick only one, this is the best and #1 in 2018.
Bitmex: One of the whales of Cryptocurrency exchange. You can do margin trading, if thats your thing.
Bitfinex: Offers mobile app and a popular exchange. You can also lend crypto here to earn interest.
KuCoin: One of the strongest exchange that also offers a mobile app. They have been constantly updating their mobile app to make it one of the best in the industry.
Huobi: Many new cryptos can be found here. Unique approach and something you should signup for in 2018. However, volume is growing.
Changelly: Perfect for quick crypto exchange. Easy to use and great for beginners.
Bittrex: Another high-quality exchange with a lot of coins.