for me in flash, it's aviary phoenix and sliderocket ...
there are many very cool javascript apps, but the thing is, far too many of them don't degrade well, if at all, which makes it a little pointless to me ... most of the time, they rely on a very specific browser, using features, that have been built in, although far from becoming part of the standard, so it's a) a bit like building a desktop app scripted with JS, and b) you have to face the problem, that the browser of your choice isn't used by the majority of users ... the google suite as it is now, is very impressive (docs, calendar, etc.) and runs in most browsers ... and personally i am quite impressed by TinyMCE, even though it is "only" a text editing component ...
don't know about silverlight ... haven't seen a real killer-app yet, only very impressive proofs of concept, which seems quite astonishing to me, since i thought many .NET apps will be simply adapted/ported to the browser, but either that didn't happen yet, or i missed it ...