Bittrex and Poloniex are two exchanges which are responsible for most of the altcoin trading volume every day. Both are well respected and there are complaints for both of them.
They exchange dozens of different altcoins, not just the typical BTC and ETH, but many new, up and coming cryptos. When it comes to Bittrex, you can trade using BTC, ETH and USDT. Poloniex has a slight advantage here as it also offers trades through XMR.
In regards to support, both exchanges are fairly dependable. Poloniex is my favorite, with a clean Q&A section on their site that can be easily accessible, and a contact page where you can quickly send a message.
On the other hand, Bittrex offers its support email on the website, they also have a Q&A section and a much wider social media presence that includes a Slack and Facebook page. Both platforms also communicate via Twitter.
Poloniex fees are very competitive, charging 0.15% to Makers and 0.25% to Takers with a trade volume bellow 600 BTC. As the trading volume increases, the fees decrease. Brittex fees are simpler but more expensive overall. All trades have a 0.25% commission.
Both exchnages seem to have had their fair share of bugs as well as multiple cirumstances where the exchanges were down. This of course is impart to the fact that this technology is fairly new and that the exchanges may at times be overwhelemed with so many trade requests. After all, we cant expect them to work like the NY Stock Exchange, which has been operating for decades.
In regards to hacks, almost every altcoin exchange has been hacked in one way or another and that is why it is always recommended to move altcoins to offline wallets as soon as possible.
It is clear that both exchanges are some of the best when it comes to trading a wide variety of altcoins. If I had to choose, after doing so much research, I would go with Bittrex. Overall it seems to be the safest and when it comes to trading cryptocurrencies that is my number one priority.