The diagnostic tools provided by the manufacturer aren't bad, although in the past they gave me a few false positives. If the drive is covered under warranty the manufacturer may even require you to run their diagnostic to approve the RMA.
The diagnostics are free to download so it doesn't hurt to have them in your toolkit.
As for third party diagnostic tools, the first one I ever used was PC Clinic, from a company based in Florida called Data Depot, which later changed its name to PCWiz, Inc. That was about ten years ago and at the time it did the job, but it wasn't updated well to keep up with new technology and apparently they ended development on it a few years ago.
I use SpinRite as well but only on the bench as it can take days (and sometimes weeks, depending on the drive and the job) as Robo and NWPhotog suggested. Lately I have been evaluating the HDD diagnostics in PC Doctor Service Center 6, the same PC-Doctor diagnostics used by HP and other OEMs and often bundled on their PCs. Since these OEMs accept the results of PC-Doctor diagnostics for replacement under warranty it could be useful in the troubleshooting and diagnosis of hard drive problems for OOW drives as well.
One thing I would suggest (again) is to partner with the hard drive manufacturer whose products you like and sell to your customers. Not only will you have access to technical resources that will aid you in both understanding and correctly diagnosing problems with their products, it will make it easier to replace those hard drives or sell new ones and increase the value of service to your customers.
I won't go into details now but I will say that there is a definite sales and psychological advantage to going onsite as an Authorized (insert manufacturer's name here) Reseller as opposed to just another service technician.
