I agree with the author of this G+ post that the easiest, most foolproof (and therefore best) method of destroying the data on a hard disk is to destroy the disk itself. Anyone in doubt should just destroy the drive.
Failure to destroy the data on a hard drive can result in profoundly embarrassing or costly consequences, as noted in these two items:
Strictly speaking, it’s not necessary to destroy a drive to remove sensitive user data. Those interested in the technical factors behind this can read this article and the linked source material: