Altcoin exchange? I will choose the one with the most volume. That is Bittrex. This exchange has been providing service for a few years, 3 or 4 IIRC. Had a great trading engine and support a lot of altcoin to trade. They also never had a major problem like hacking or any other shady issue. Their customer support is also good, never ignoring any customer complaints.

The next candidate would be HitBTC and Poloniex. I never use HitBTC so couldn't really tell about it, but I use Poloniex frequently. It's a great exchange with a great volume overall. But there has been issue with their customer support where they ignore a lot of user complaint and left them unhandled for weeks. That issue probably isn't happening now, but it's worth noting.