Has your daughter started any solid foods? Food allergies (especially dairy) can be a cause of recurrent diaper rash. If she is eating solids, I would start the introduction all over again...one thing for several days at a time. Also, eliminate juice if she gets it.
If you breastfeed, consider eliminating dairy (or other high allergy foods) from your diet for at least 4 weeks. It takes that much time to clear everything out of your systems.
The hairdryer was our best defense against diaper rash. Use it (carefully) to make sure the area is completely dry before applying any ointment. I didn't have to use hardly any cream or ointment until she was almost a year old because I always made sure she was completely dry between changes.
We've used a combination of cloth and disposable diapers. The brand of diaper (or detergent) definitely made a difference...none of the store brands we tried worked. My daughter would develop a rash within the first diaper change with store brands. Pampers and Luvs worked best for us.
I'm surprised that the pediatrician isn't a little more concerned. If it is a yeast infection, and you are getting nystatin, one that goes on that long could be a symptom of an underlying issue. You might consider a second opinion.
I don't believe it is healthy for our babies girl parts to always be slathered with some sort of ointment so I try to figure out the cause.