Kraken number one always working never problems.
Kraken is the best one so far, it is old and never had any problem when dealing with this exchange so far. There were problems in and out but they cleared it pretty well so good job with that.
Poloniex old and helpful support.
They have a great support. I had a coin where I sent it to a wrong address and they were willing to move it manually for me, props to that. But, after they implement the KYC that makes me quit to trade on this exchange.
Binance well... They should work with their down time problems.
Haven't encountered this issue so far, although my ISP is shit and choose to block this site for no reason lol(thank god I know how to resolve this issue)