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I have used Poloniex ( https://poloniex.com/ )too which although I had some issues in the beginning, they appear to be one of the best out there. The other one that gets a lot of recomendations on the web appears to be Gdax which is Coinbase's exchange. Coinbase have had a ton of money pumped into them last year so they are building up a strong, sustainable company now. https://www.gdax.com/
I haven't used Gdax myself as yet as there was no need with Poloniex and Bittrex on hand. I would recommend you read through the fees pages really well because it is something that ever changes due to volatility of crypto currency's - this means some put up their withdrawal fees and mins and maximums - this caught me out a few times trying to withdraw too little amount and being stuck with an amount of a coin I could only trade. I hope this helps, and please do shout up with any further questions or queries and I shall try my best to point you in a direction of information worthy of reading.

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