I personally use Bittrex almost daily.  Very fast to process transactions, good exchange rates and high volume.  I have also used Poloniex and Bitfinex a lot.  I do not recommend those exchanges.  Poloniex has had some negative press from freezing accounts that soured me.  And with Bitfinex, frequently I would deposit a specific amount of BTC to trade for another coin (I don't like holding funds on exchange, only deposit to buy something else then withdrawal).  Far too frequently when I try to buy max, I end up with a negative BTC balance of like 0.000003 or something ridiculously small, and then they say they won't process your withdrawal because of a negative balance, so then you have to sell some of what you just bought (and their minimum trade size isn't quite to 'minimal') to restore your negative BTC balance. So I've decided to just leave like 0.0005 BTC on their exchange so this doesn't happen again.  It's pretty crappy that users have to troubleshoot their exchange like this to avoid problems.  Why the hell do they let you sell more BTC than you have?