You can have a look at Bittrex here
www.bittrex.com and Binance here
www.binance.com I like their security level for both of them there are so good with further security measure of 2FA which will send some signal to your personal smartphone before any access will be granted to your wallet with either of them.
Yeah they are quite secure, but I saw somewhere else on the forum (I can't remember where exactly) that someone was testing the security flaws of exchanges like theirs. They found quite a few holes.
Most exchanges have some kind of 2FA feature now and it is a good feature, but I believe there are still backdoors in. Not necessarily to your specific account, but to the exchange itself. A hacker who gets in through another way can then get into any of the accounts quite easily. Although I would assume that because they are such large profile exchanges, they'd have someone on fraud alert 24/7 like other major payment gateways do.