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.
You are right on your answer mate, I was using bittrex for a years I think , then I've been in Poloniex too before but I pulled out all my coins in Polo during the time that they faced some issues, moreover when Binance, kucoin, Coinbene, Houbi and okex came out I shifted on these exchange now. Why? it is because those are listed at the top exchange platform in the market where Bittrex and Poloniex has been surpassed by these exchangers. Though I'm still using bittrex now but not as often like what I did before.