It's very easy to choose top exchanges using coinmarketcap today since they are rated based on liquidity unlike last year when exchanges are rated by volume and some exchanges like p2pb2b exchange that have fake volume was among the list of to exchanges which is very misleading
Exchanges need to be tracked not only by trading volume but also by user reviews. Trading volumes can be made using bots and real trading volumes can be significantly lower than indicated on the coinmarketcap.