Best language/runtime for desktop app

What would people say is the best runtime (i.e. JVM) and language (i.e. java/jython/scala/python/C++) for building a desktop application for scientific data manipulation and visualisation? Ideally the end product should run on at least Windows and Mac (Linux is an added extra)

I'm currently considering the pros cons based on: - Runtime performance - Conciseness of code - 3rd Party Libraries (not neccessarily in the order)

Ideally something like what adobe was doing with flex would be ideal, as you could write a (client) app and deploy to: - Web (via flash) - Desktop (via Air) - iOS/Android (again via Air runtime)

But since Flex may be unsupported now, I don't want to invest heavily in the technology.

All comments welcomed.

Thanks, Vackar