Ask HN: which is the best flash video player?

JW FLV Player.

http://www.longtailvideo.com/players/jw-flv-player/

It's the one you see a lot of sites using.

You mean http://flowplayer.org. In my personal opinion flowplayer is excellent. It has the most extensive customization and API of any of the available flash players. Especially if you're using their most recent 3.x version. Furthermore, they have an excellent set of documentation and examples.

JW player works well for playing videos, but if you plan on using the plugins be aware that many of them conflict. http://corp.kaltura.com/ is another option.