The one that you understand how it works and know what are its weaknesses.
Buut quickly, here, i'd say:
- must use asymetric encryption to derivate the AES key; each binary must have its own public key, so private key to decrypt is never on the memory/hdd of the victim
- must also take care of network shares and backup, shadow copies, etc.
- why not encrypting partial sections of file, enough to make it unrecoverable but small parts so encryption is damn fast