Guess technically binance. You have access to USDT and you can convert leftover dust (unsold fractions of a certain coins) to bnb, which effectively allows you to pay half the fees. By far my favorite exchange. Bittrex was on top at some point, mainly cause it has a tonne of obscure coins.
Binance is in fact a good exchange. It is great for beginners, since it has a fairly simple interface. Also, it trades a large number of good tokens, it is reliable and fast. But for professional traders it is not the best option. After all, you can not open short positions on it and it has no margin trading. Such functions can be used on Bitmex and Bitfinex exchanges. So I suggest to pay attention to them.