As much as I would have been tempted to pick many of already know centralized exchange as the best exchanges, the future of cryptomarket is not with all these centrlized exchange. Decentralized exchange is the future and in that space I will pick etherdelta, Idex and forkdelta as best exchanges!
Centralized and decentralized exchanges have two very different use cases. One is really good and efficient at facilitating high capacity trading while the other is rather slow and expensive, good for long term investments and low counter party risk.