for us also the biggest and also reliable, for us you can trade in market .hitbtc, binance, poloniex and also in bittrex, for us also will easy to get profit, and also many new coins there.
poloniex is not a reliable exchange. I have long withdrawn my capital from there. You of course can get lucky and you will not experience problems but my rule to leave the exchange after the first signals that users periodically have problems. Now really a lot of exchanges but there is no one reliable and universal. It seems to me that this market is still open for honest business.