which is better ? adobe flex, ajax+dhtml, flash mx, or silverlight + xaml?

Which is better -- a screwdriver or a hammer? Well, it depends on what you are trying to do and what you are trying to do it with. Both put metal in wood, but you wouldn't use a hammer to drive screws and you wouldn't use a screwdriver to pound nails.

Every tool has advantages and weaknesses. What do you plan to do with these tools? What are your limitations and requirements?

I like Silverlight. You can view 6 hours of video tutorials and much more at: http://idealprogrammer.com/languages/silverlight/s...

once you install flash participant the installation takes an extremely couple of minutes... if the movies in youtube are actually not yet working yet you have deploy flash participant attempt to acquire yet another one and deploy it....