I have an account with Bittrex, Poloniex and Binance.
Poloniex had some issues before I opened an account. Since September 2017, it looks fine to me, never had any problem.
Bittrex had some issues by the end of 2017. I had an enhanced account, then they blocked my account, I couldn't withdraw any money because of a name mismatch that they created for many customers. It took months to fix the problem, but it looks fine now.
I have a course on Udemy, I tried Binance for my students because I wanted to give some alternatives to Bittrex and Poloniex. I must say that it seems to be the best one.
I still use Bittrex mainly because it gives you so many choices, but Poloniex and Binance look good to me.