If it has no bad sectors and no HPA, or if you had the foresight to encrypt it: One dd zero-pass then throw it in the bin. You're done here.
If it has had bad (remapped) sectors, or has an HPA:
hdparm -I /dev/<insert device here>
# check it's not frozen - if it is: echo -n mem > /sys/power/state # then wake the device, try again - it'll probably be unfrozen
hdparm --user-master u --security-set-pass password /dev/<insert device here>
time hdparm --user-master u --security-erase-enhanced password /dev/<insert device here>
then one dd zero-pass just to make sure, then throw it in the bin.
Nobody, not even the NSA actually investigating al Qaeda, has ever been able to recover past this with an untampered drive. It's completely impossible, even with electron microscope techniques, to get any kind of statistical confidence with modern perpendicular-recording drives (let alone SMR). That 26-pass bullshit you heard of was from ancient MFM/RLL drives, go and ask Gutmann himself if you don't believe me. If you can get anything at all, it's going to be maybe one or two random clusters from remapped sectors: but enhanced secure-erase tells it to wipe those, too.
Note that SSDs fail differently to HDs: blocks either fail erase or become 'sticky' and can't be properly written, instead of becoming unreadable or unreliably written. You cannot securely erase NAND Flash memory, or even NOR Flash; only old-level wide EEPROM. The secure-erase technique above will however tell the drive firmware to at least try to erase the blocks. If you can't do that for some reason, blkdiscard /dev/<device> tells it to trim all sectors by default. That won't be perfect, but it'll do.
You should encrypt confidential or personal data rather than try to destroy plaintext.
You can also take the drive apart if you want handy neodymium magnets. You'll usually find them in there, being used to keep the heads from crashing into the hard disk if it suddenly loses power.