I use exchange with quality definition of security feature to make sure my investment will totally secured, and of course aside from that it's convenient to use buy and sell coins for my trading job.

For a good crypto exchange, should fulfill the qualities which are needed these days.
First point is security. Now a days we are seeing lot of cyber attacks on renowned exchanges which are stealing users hard earned money. I believe every transaction should follow with 2FA and their firewall should be strong enough so that it can avoid any kind of cyber attacks.
Second most important thing I see is fee. Many exchanges charge too much fees making it impossible for small traders. They should reduce the transaction fees to great extent so that more users can use it.
Third important thing I see is availability of coins to trade. There are very few, one or two exchanges I guess which support most of the altcoins for trade.

Very basic, What attracts me is the their liquidity pool, It creates some big trust of the liquidity pool of the exchange is huge, like, they might have the enough support to their crypto in the market so I can let my money flow through it and provide me with a decent income or profit for an investment through trading.