My vote is for Python. My reasons are that it'[s very good for the rapid part. There's also tons of libraries to do darn near everything under the sun (see pypi.python.org). Finally, one thing in their mantra is that readability counts. This means that you can pick up your project several months later and know what it does... maybe even someone else's! Try doing this with Perl or Ruby, and it's much harder.
Python works quite well on the UNIX like systems, decently on Windows, has good command line helper libraries (argparse or optparse), and has several really good web frameworks. Heck, you can use IronPython or Jython and mix into your .NET or Java code!
The biggest weak point is probably full GUIs. It's not that there's not any good ones, there's just not a good default one. TkInter is built-in, but it's based on Tcl/Tk, the interface isn't very Pythonic, and the end result isn't great. WxPython is good for a basic GUIs, but adding custom widgets is hard. PyQt and PySidehas a more complete collection of widgets, but it again is tough to add new widgets. PyGTK has the large collection of widgets, and widgets can be written in Python and become first class widgets even in other languages. The new kid on the block is Kivy, which is kind of like QML for Python. Kivy defines very low level functionality that builds up widgets, but it makes it easy to combine them together to make a complete widget. This sounds like a lot of work, but it turns out to not be as bad as you'd expect.
Also, PyDev, PyCharm, and WingIDE are all pretty amazing IDEs for Python.
Finally, there's a good amount of jobs asking for Python, especially in big cities.