In my own opinion there are criterias to focus on while saying this exchange or that exchange is a top cryptocurrency exchange be it centralized or decentralized although most decentralized exchanges are still struggling. However to classify an exchange as a top exchange the first thing to put into consideration is security followed by volume then users. On the basis the first exchange I can mention is Okex, then Binance, Huobi, Gate, Kucoin and lastly Bittrex.