I'm aware of FusionCharts, are there other good ones?
I know you said flash, but this is a good silverlight chart api. http://www.visifire.com/. Always good to keep your options open right?
Had to plug silverlight, but you can also take a look at Yahoo!'s YUI charting component.
Is there a reason that you want it in Flash? If a plain, old PNG will work, try the Google Chart API.
I can vouch that I've had good experiences with PHP/SWF charts (and by extension, the XML/SWF charts too). It's easy to create really subtle chart effects. Pie charts fading in one slice at a time looks fairly professional without being annoying.
Edit: Scratch that, Open-Flash-Chart looks WAY better. I recede my suggestion.
http://teethgrinder.co.uk/open-flash-chart/ seems really sweet. I suggest you give it a look.
UPDATE: Open Flash Chart 2 is out: http://teethgrinder.co.uk/open-flash-chart-2/
I think the best Flash charts are amCharts - nice looking, highly customizable and free (if you don't mind the link back to amcharts.com)
open-flash-chart because you can't resize a .png and .png has no tooltips. Also why would you send all your data to google? Do you trust them that much?
Adobe's data visualization package is now free after the release of Flash Builder 4.
Charting and other libraries are on this site - http://opensource.adobe.com/wiki/display/flexsdk/Download+Flex+4