Bittrex and Bitfinex are the ones I currently use but like some of the others have already said, use Bittrex for its wide selection of different crypto-currency pairs since it has a lot of the one thats can 10X your money and use Bitfinex for all the main Crypto-Currency Pairs since the platform and interface on Bitfinex is so much easier to use with more useful tools. Another idea would be to sign up for Coinigy and import your API Documentations from every exchange that you already have so you can trade off of one website, one platform and have your information of your P/L all on one website. I think they still have a 1 month trial so can check it out. They seem very professional and always adding and improving new features to their services.