I'd say the exchanges that currently remained to be unhacked are pretty "safe"; probably Binance and Coinbase. But that doesn't mean I'll recommend leaving funds on these platforms. The developers can make the platform as secure as possible, but never secure enough to the point that it's impossible to hack. Exploits will always be a risk. Not because these "unhacked" exchanges remained unhacked for years doesn't automatically mean it'll stay the same forever.
I don't feel like I really want to agree, but yeah, Coinbase and Binance probably what you'd call the safest you can find. Coinbase because they've probably got the most robust licensing (and are a principal partner for the debit cards they issue, which is something you don't get unless you open your books to them). Binance because of their reserve fund (but I can't say I've ever verified for myself this proof of reserve).
I would say one day we'll see an exchange or at least a financial institution with a crypto exchange arm that'll get a true banking license along with the deposit guarantee schemes that all national banks get from their respective countries. Then I might say that's as safe as it gets (since with that guarantee it also means you open yr books to the central bank, though we all know that's not much in way of safety these days).