What's the best converter to use for convert video file(s) to .flv with .swf player?

Here is a few you can try out, no one can know what you think is the best:

Windows
Will contain watermark on the free version: http://www.flash-swf-converter.com/features/swftoflv.php?ref=google

Not sure what the catch is on the free trial just yet (perhaps its is limited to .swf to .flv?): http://www.swfkits.com/swf-to-flv-converter/download.htm

Free trial, not sure what the catch is here either: http://www.swfflvconverter.com/swf-to-flv-converter.html

Adobe Flash Professional isn't strictly a converter, but if you already happen to have it, you can use it. I don't suggest buying it now unless you were planning on it anyway though, its rather expensive compared to the alternatives: http://www.adobe.com/products/flash/

Mac (if you have an Intel Mac, you can run Boot Camp or Parallels to use the Windows programs as well)
Free trial here as well: http://swf-converter-for-mac.mp4kits.com/swf-to-flv-for-mac/

Adobe Flash Professional isn't strictly a converter, but if you alreadyhappen to have it, you can use it. I don't suggest buying it now unlessyou were planning on it anyway though, its rather expensive compared tothe alternatives: http://www.adobe.com/products/flash/

Hope that helps.

You can use a website callled http://vixy.net/

or you can get a software called Magic Video Converter

I think you can transfer video to FLV flash video with this video to flash tool.

No best one, different people has different idea. It's really depends on your requirment. If I were you, I'll use iFunia video converter, it has Mac and Windows version.