Bittrex and Binance are definitely two worth a mention. But newer exchanges like Kucoin, Huobi and Cobinhood (going to be a decentralized exchange in the near future) are definitely worth a look, if only because they seem to get some of the newer coins on quicker. There are hundreds though, all varying in terms of ease of use, coins available and reliability. Don't think you would go wrong with the five mentioned above though.