About half year ago I would say Poloniex all the day... Now I feel like best is Binance, they have own app which you can use when you aren't near pc/nb. They have low fees with BNB, they do a lot of marketing and number of traders rising. Bittrex is like neutral, it has more coins but I don't like it that much as Binance. Good is also Kucoin, they seems to me like just copying Binance model.