Hi, I would say this is too broad a question and it's answer is outside the scope of this forum. Choosing a server side technology will depend on yours or your clients requirements or the skillset you are working with.
I'm not sure if Asp.Net or Java would be better suited to Flex but if you want to work with a server-side technology that will 'probably' offer better integration then you may want to look at ColdFusion as I believe it supports AMF.
Having said that, if you want to hook directly into your RDBMS, then you may want to look at 4D <http://www.4d.com/> in particular 4D for Flex in the 4D Web Pack... http://www.4d.com/products/4dweb20pack-FL.html
Hope that helps in some way...