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.
If good exchange is that operated more than 2 to 3 years. How about Binance, okex, and houbi these 3 exchange as far as I know surpassed Bittrex and Poloniex already and these 3 exchange platform also listed on top 10 exchangers. Probably, in choosing a good exchange I think the good basis is the volume itself they have. Meaning, years is not a basis for us to know the good exchange.