What do the collective beleive to be the best platform to use as a backend to AJAX / Flex / Silverlight applications and why?
We are undergoing a technology review and I would like to know some other opinions.
Is It Java, Grails, Python, Rails, ColdFusion, something else?
There is no definitive answer. However, I would choose a light solution, like Python or Rails, over Java or ColdFusion.
You may want to investigate C# ASP.NET + Silverlight combo. Microsoft made it highly integrated, which is double-edged sword. But in many cases this helps.
You may also want to review existing solutions / applications / startups. Don't ditch PHP up front, there are many existing components for it. And don't overestimate the impact of server-side technology choice on success.