Let me guess you are as a newbie and then you need exchange site that is very trusted without a lot of restriction to the users, I just find binance as best exchange right now and okex can be considered as alternative service from binance, kucoin should on 3rd place after binance and okex. But it looks hitbtc already actively to resolved all of their problem. I would never give any recomendation to use bittrex or polo.