"Delete" or "Format" doesn't mean Erase. Data Recovery experts can read data from the disk surface of a hard disk that has been burned, immersed in water etc, by mounting the platter in a special machine that reads & reassembles the data bit by bit.

The best way to securely delete the data will always be by using a software utility that meets or exceeds DoD 5220.22-M or 5220.28-STD, or Gutmann standards. In these standards, typically every byte is overwritten a minimum of seven times by a random character, followed by it's complement. this means that at the individual bit level, the zeros and ones have been toggled more than 7 times, rendering it guaranteed completely unreadable, and any 'ghost' of the previous bit value well and truly obliterated.

If you just want to make it 'difficult' for someone to mount your working hard disk in another computer and read it, then follow any of the dramatic & fun ideas-

-"physically" destroying
Drill it a few times.
Sledge hammer route.
BLOW TORCH.
Drill a hole in case and fill with acid.
Remove the platters from the disk.
powerful electro-magnet.
Use a grinder and grind the drive into dust.

-"digitally" destroying
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Some Linux boot disks have tools like.
Low Level Format.

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-keyword (killdisk)