The safest trading Bitcoin trading platforms are the ones that have the best security measures.

Crypto platforms have two ways they can store their customers' assets, one is hot storage where they save them online and can easily access it and is not very expensive to maintain but very prone to hacking by scammers.

The other way is the best method of saving assets which is the cold storage and that is storing them offline away from the internet. This method is by far the best means of storing cryptocurrencies, as it's out of reach of potential scammers.
Platforms savings assets online at some points might have experienced hack attacks but exchanges saving offline hardly get such, because of the security nature of it.

You can look for exchanges with cold storage and use, and an example is Quidax which saves assets offline and has zero history of hacking.