(Hmmm - maybe I should have posted this as a 'comment' rather than an 'answer' - sorry, still getting the hang of the board...)
QT is working on providing a full web application framework, but it's very, very embryonic. You can find out more info, here: http://labs.qt.nokia.com/2010/07/19/the-qt-web-runtime-journey-begins/
The plugin technology is mature - based on the NetScape plugin protocol - and works well, but, of course, you have to have separate plugins for Mac/Win/Linux, other OS's. Qt helps a lot in that regard, but you will have to maintain separate builds and deliver them appropriately. You'll get the best performance with Qt since it, like Active X, is C++ based.
Flash/Flex is a good development environment (Eclipse-based), ActionScript is kind of a funky language - it's essentially an ECMA-based language (JavaScript-like) with stronger typing. It's ok. You really have to get the event sequence down to know when you can rely on sizes - Flash defers everything it can until later. Flash is ubiquitous (unless you're a iPod/iPad devotee....) The big downside of Flex is the executable images are HUGE, no matter what it does...
Silverlight uses C#, which IMHO is a really good language. The executable images are much leaner, presuming you're not using the entire SDK. Downside is it's not available on a lot of devices, AFAIK.
Don't know much about HTML5 - it's ultimately a good direction, but you'll have to deal with browser support. If you have a requirement to support old browsers, it will be an issue. But, you'll be able to support a lot of devices, particularly iPod/iPad, and will be well positioned for the future.
Not providing a choice, here - just bringing up some issues you may or may not have already considered...