The choice of the exchange depends on crypto assets in which you expect to trade and also from your personal preferences concerning methods of input-output of the means raised for transactions of commission charges, conveniences of the interface, etc. If to consider BTC, ETH, and other most popular tools, then in respect of liquidity, opportunities for trading, functionality, and convenience as the leader, perhaps, it is necessary to designate the Bitfinex exchange. On it the very friendly interface, marginal trade and a set of types of warrants are available, and recently on the summary trading volumes, only the Korean Bithumb can compete with it. Though to be necessary to mark that at some traders the exchange arouses mistrust because of implied communication with the Tether company which is releasing USDT and also in connection with complaints on platform failures during a big agiotage in the market which, however, are inherent in all large cryptocurrency exchanges.
Also for trading on the most liquid crypto assets, it is possible to pay attention to Bitstamp (one of the oldest and, perhaps, the most reliable exchange in respect of regulation as possesses full licensing in the EU), Kraken and GDAX. All three have smaller liquidity and are a little less functional in comparison with Bitfinex, and on Bitstamp marginal trade is also unavailable.
For trading, the Poloniex and Bittrex exchanges registered in the USA remain less liquid tools still very convenient and reliable. Though recently users complain about the need of passing of verification for a raising of limits of an output to them and also the absence of placement of the new appearing cryptocurrencies. For this reason (thanks to the placement of new perspective cryptocurrencies, loyal conditions, and usability) the blanket on itself was a little outweighed by the Binance exchange registered in China which after prohibitions National Bank of China switched completely to service of users from other countries.