It depends on what you want to trade: if you want to trade a lot of different cryptocurrencies and don't need leverage, you can choose binance, hitbtc, okex or huobi. If you want to trade some top cryptocurrencies (btc, eth and something else) with leverage 5x/10x (up to 100x, but 50x/100x is like suicide, especially now) you can choose bitmex or bitseven.