Best way to wipe data on HDD

You can certainly check out Dban.  
https://www.pendrivelinux.com/install-dban-to-a-usb-flash-drive-using-windows/

You do not tell us what kind of specifications.  Do you need to certify these erased drives or have a certificate?  How crucial will the erasure need to be?  Do you expect to have little to no recovery available to the drives?

I've heard good things about all of the tools (Scrub, Dban, and DiskWipe) but haven't used any of the three.  I've used ActiveKillDisk http://www.killdisk.com/ and it works great.

If it is easy enough to remove the drives, it may be quicker to connect multiple ones to a single PC and wipe them at once.

Similar discussion took place here: https://www.experts-exchange.com/questions/29130736/Format-and-erase-data-off-hard-drive.html?notificationFollowed=220774408&anchorAnswerId=42764123#a42764123
Have a look on it.

Could I use the free version of Kill Disk?  

Also, does it matter which method is used to wipe the disks?  DOD/One zero pass, etc

Are they going to reinstall an OS on the drives or are these going to be disposed of?

New tenants are going to re-format/re-install OS.

DOD is considered to be secure. In different countries different standards are prescribed.

A couple dumb questions

1) will the HD be unformatted after one complete zero pass?

2) will an OS be able to be installed on the HD after one complete zero pass?

1) it will look like as if it were new. No partitions, no file system.
2) Yes. OS install wizard will create default partitions it needs for installation.

And one final Q:

1) is one zero pass less secure than DOD?   Is it feasible that someone will revover data from one zero pass and not DOD?  

Do I need to even worry about the erase/wipe method?  Basically I am wiping all windows config/proprietary info off of PCs that are being acquired by another company.    The new company will format and setup the PCs for themselves.

It depends which data your drives contain. If there is no sensitive data on the drives then just format and delete the partitions. No erase steps from your side are needed.
If you or anyone in the company do/does not want that someone gets some data from these drives then you have to erase it.
What is DOD? It is several passes of writing different values into each sector.
I have seen cases where after single writing zeroes the data was restored.

Why to use some special software if all tools you need it's build in in Windows.
Try to format disk 3 times with 3 different allocation unit size, then run some unformat software or Get Data Back software and try to recover any data ,...

Good luck

@noxcho:
"I have seen cases where after single writing zeroes the data was restored."
I've wondered about this for quite a while.  Much of what I've read online implies that this is unlikely/impossible with newer (last 10 years or so?) drives.  How old was the drive on which you saw this?

5 - 7 years. I worked for a company which produced wiping software (writing zeroes or any other value, pretty similar by all wiping tools) and we got a case where the data was recovered after single run. The only solution was using several runs though it drastically slows down the erase process.

Interesting....  Am I correct that it involved special hardware to read residual data between tracks?  That's the usual method mentioned for older drives.

Nope. The customer used recovery software with internally connected drive. We were able to reproduce the problem with their software as test.
The modern technologies are even more advanced. The drive plates have on them rest traces of magnetisation even after the data is erased. There are tools used by special departments to restore data for special purposes.

As noxcho suggested the only reliable way to erase all traces of data from a disk is to use a utility like scrub, where you write various patterns of data, repeatedly across the drive.

Davis' reference is the sort of thing I've read in recent years.  It is in direct conflict with noxcho's first-hand experience, if I'm reading all of this correctly.

This reference for a government is written by some IT specialist. And he shared his knowledge. What I saw with my own eyes was a problem which a client (a financial institution) reported and was confirmed. The DOD standard and similar standards from UK and other countries are not done just for fun ;)

It’s probably quicker and cheaper to just buy brand new disks and put the old ones in the chipper

If one had very expensive and highly specialized equipment up until about 2003, it was possible to deliberately move the heads slightly off track which resulted in the ability to recover a very small fraction of the data which had subsequently been overwritten.  However; with the changes in the physical structure of hard disk drives and recording techniques, the return on those efforts has effectively become zilch such that a single pass of writing quite effectively destroys the previous data.  Bear in mind that the new user of the drive would have to know they wanted the data from that specific drive and was willing to spend several tens of thousands of dollars to get a small fraction of 1%.  Unless you are a cartel kingpin, there would be no gain.