There is a lot of rumour and development in the RIA world. Microsoft recently released SilverLight and Adobe Flex is nowadays an established and popular RIA development environment.
Today, I read that Sun roles out it's own RIA development product. JavaFX is Sun's answer on Adobe Flex and Microsoft Silverlight. JavaFX is a product-line that can be used for building RIAs rich web/desktop apps all the way down to to handheld device applications. FX script will become the tool to for creating your RIAs. The first product that will be releaded is JavaFX mobile, the kit for developing RIAs for mobile devices
FX script is the tool for content-authoring with a main focus on UI experience and making the RIAs highly animated. FX script make is possible to use every library in SE or ME. No bridging-code is needed. FX script will deal with this. To use FX Script you only need to install one library with the standard SE or ME installation.
With JavaFX, Sun now has its own Java-based RIA platform to compete with Adobe and Microsoft on the RIA market. According to this, there are three main players now on the RIA market: .Net, Adobe and Sun. Forget to mention AJAX, but in my opinion this is more a technology than a product for building RIAs. Also, Grails and Ruby on Rails in combination with AJAX are very usefull frameworks for building RIAs.
I believe that accessing, displaying and more important sharing non-structured data, like sound images and video on any platform is now very important and will become more important in the near future. So the RIA market will also emerge in the near future. The more players enter the RIA market the more choices we will have to develop RIA applications. I do not have an appropriate answer on the question which product is best. Maybe, there isn't and will the choice depends on your situation and project demands.