No need to look further, we have big exchanges with high liquidity so no need to look for more. I personally use binance, bittrex and hitbtc.
I list some though these are common things that we should look up to.
Privacy
Security
Liquidity
24/7 support
Instant deposit and withdrawal
This is a good outline of what and Exchanger really need to acquire and also the trading fees and withdraw fees should not be higher.