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 don't know what are those exchanges in cryptocurrency that operates 2 to 3 years but maybe, it is one of the reason we need to see if a certain exchange is legit or not. There are many exchanges that exist nowadays and the worst is, most of them are scam exchange. If I will trade my coins, I always wait for a centralized exchange or popular.