If security is your biggest concern, and not reuse... then using industrial shredding machinery is your best bet. If you want reuse... then a handful of fill/e

If security is your biggest concern, and not reuse... then using industrial shredding machinery is your best bet. If you want reuse... then a handful of fill/erase cycles with random data is probably your best bet, even then, there will still likely be some overprovisioned blocks with prior data. Encryption and throwing away the key doesn't do more than random data to fill said drive.