Good exchange is exchange that operated more than 2 or 3 years and they has good reputation, never get any hack and comply with the regulation in the country their located. I used Bittrex and Poloniex more than 2 years and i have balance in that exchange also since 2 years and untill now that my balance is safe. So this both exchanges is one of the best exchanges.
I'm also using Bittrex as my cryptocurrency exchanger, its good, fast and reliable. So when choosing an exchanger, make sure that they are operating legally, safe and secured it is very essential to all user of the exchanger when it is safe and secured, hack-proof and system error=proof. Recently i used Bancor for a single transaction and i am impressed although fee is quite higher, im looking to use bancor as my optional exchanger.