SlideShowPro is the best Flash slideshow player for WordPress
Back before it was “Un Cool” to use Flash I used a flash component called SlideShowPro for Flash. It is very extensible and uses an XML file to load the content into the slideshow. It also allows you to customize all aspects of the display using Adobe Flash CS4 or CS5.
It also has the ability to auto load pictures from a Media RSS feed and there are a number of PHP scripts available out there that will generate the XML on the fly for any images added to a folder.
I just checked the website and they have brought the product along in the last year or so. It is now generates slideshows for the iOs and Android mobile devices.
The slideshow player works with the SlidePress WordPress Plugin and there is also the NextGEN Gallery Slidepress XML Plugin which creates the proper XML file for your NextGen slideshows.