What is the most unhackable crypto platform?
No exchange is immune to hacking; once hackers discover their vulnerability, they will use it against them. The more security measures put into protecting users assets, the more hackers are also investing more time in upgrading their tools.
The only good thing about decentralized exchanges is that your funds are not stored in them, so even if they are hacked, you will only have little to lose since you will have to disconnect your wallet from the DEX when you are done using it. It's only when your funds are still there that hacks will affect you, like what I read about Etherdelter.
If hackers can penetrate smart contracts and create their own token or do whatever they want with it, to what length do you think they can go? Some exchanges have stronger security than others, but that still doesn't make them immune from being hacked.