Which exchanges would you suggest for security? Which ones to avoid? How to tell a good one from a bad one?
None of the centralized exchange platforms are secure to use but if they are registered than you can file complaint against them. Poloniex, bittrex, coinbase, bitstamp all are registered trading platform but they can froze,block your account without any reasons if you trade large amount of bitcoin.
There is one bitsquare which is decentralized trading platform but there is not much liquidity but you can trade ETH based tokens in etherdelta which is another decentralized trading platform.