I have experience in game development in some game engines in Action Script 3 and C++. However, I would like to improve the productivity and so I want to develop a new project in Python, ruby or LUA. Would it be a good idea? If yes, which one would you suggest? and what is the killer game development tool set or engine?

If you're any good, go with Pyglet.
It's a cross-platform Python version independent hook against OpenGL with outstanding performance. It's a bit tricky but it does the job better than anything else out there in the Python world.

If you're a beginner, i'd go with Pygame.
It's a bit taxing on the system but with a modern computer that isn't a issue.. also, it got pre-packaged API's for game development (hence the name) :)

A "official" list of Python gaming/graphic engines: http://wiki.python.org/moin/PythonGames

Some good ones:

• Panda3D
• Pyglet
• PyGame
• Blender3D

Example Pyglet code:

#!/usr/bin/python
import pyglet
from time import time, sleepclass Window(pyglet.window.Window):def __init__(self, refreshrate):super(Window, self).__init__(vsync = False)self.frames = 0self.framerate = pyglet.text.Label(text='Unknown', font_name='Verdana', font_size=8, x=10, y=10, color=(255,255,255,255))self.last = time()self.alive = 1self.refreshrate = refreshrateself.click = Noneself.drag = Falsedef on_draw(self):self.render()def on_mouse_press(self, x, y, button, modifiers):self.click = x,ydef on_mouse_drag(self, x, y, dx, dy, buttons, modifiers):if self.click:self.drag = Trueprint 'Drag offset:',(dx,dy)def on_mouse_release(self, x, y, button, modifiers):if not self.drag and self.click:print 'You clicked here', self.click, 'Relese point:',(x,y)else:print 'You draged from', self.click, 'to:',(x,y)self.click = Noneself.drag = Falsedef render(self):self.clear()if time() - self.last >= 1:self.framerate.text = str(self.frames)self.frames = 0self.last = time()else:self.frames += 1self.framerate.draw()self.flip()def on_close(self):self.alive = 0def run(self):while self.alive:self.render()# ----> Note: <----#  Without self.dispatc_events() the screen will freeze#  due to the fact that i don't call pyglet.app.run(),#  because i like to have the control when and what locks#  the application, since pyglet.app.run() is a locking call.event = self.dispatch_events()sleep(1.0/self.refreshrate)win = Window(23) # set the fps
win.run()

Note on Pyglet with Python 3.X:

You'll have to download the 1.2alpha1 otherwise it will complain about you not having Python3.X installed :)