Client side programming preferences (AJAX, Javascript, Flash/Flex, Silverlight)

I am a huge fan of simplicity when it comes to web development. But I have come across the opportunity to develop an application that needs to be rich in interactivity. Extensive video manipulation, form interactivity, tracked user progression through interactions, and a generally engaging application are all characteristics a rich application might have. Adobe coins them RIA's, rich internet applications. There are a lot of drawbacks to using heavier "machines" on the client side, such as Flash Player and Silverlight, and AJAX and Javascript tends to be MUCH lighter, scalable, and portable across platforms. How much capability do these lighter scripting languages have to lend to the development of rich applications?

The question is, what client-side language would the provide the perfect balance between rich interaction and performance? If anyone has a little more experience with these languages, maybe a post comparing these frameworks can shed some light on the topic for me and any developers following in my footsteps.

P.S. I am going to follow up with a "server-side" post later in the week.

~Eric

You know things like Google maps are all ajax.

http://developer.yahoo.com/yui/

that might help as well. The library has everything you need really, but... learning it will take you awhile.

http://gallery.yahoo.com/

Gallery of examples. Really, if you know how to do it, ajax can do anything you'd want for a productivity application.

http://www.themaninblue.com/experiment/BunnyHunt/

Game... ajax, not flash.