Poloniex is good. I believe a couple of years ago they were hacked and lost a fair amount of bitcoins. I think I lost around 0.5 bitcoin at that time. They paid me back and everyone else as far as I'm aware and they have been smooth sailing every since. I've used mt-gox, cryptsy, bter, btc38, btc-e and some smaller ones and right now poloniex is my go to exchange. Although I did get hacked (email) and lost 1 btc worth of crypt/bitshares from polo exchange they were apologetic but said it was my fault for not using two-factor authentication. Which seems fair.