What are the best apps for rescuing deleted data on flash drives?
I recommend ProSoft's Data Rescue which can recover a very wide variety of file types. Even if you don't see the file type listed, ProSoft maybe able to help you recover the data. I once had a client delete a their QuickBook's for Mac data file. It was a new version of QuickBooks with a radically different file format from prior versions. The current Data Rescue didn't recover the data; however, ProSoft was able to provide a new file definition and all of the data was recovered. 

There are separate Mac and PC versions of Data Rescue. I have used both with good results, but I have far more experience with the Mac version.

I find $99 for Data Rescue to be a very fair price, however, if you are only needing to recover image files from digital cameras, checkout ProSoft's Image Rescue for $20. I believe Image Rescue comes with both Mac and PC licenses.