According to the IMF it is Saudi Arabia:
Saudi Arabia responded to the crisis with the largest fiscal stimulus relative to GDP in the G-20, which the IMF deemed appropriate. The IMF website forecasts real GDP growth to be 4 percent for Saudi Arabia in 2010.
The International Budget Partnership (IBP), an organization dedicated to evaluating fiscal transparency in countries throughout the world, gave Saudi Arabia a score of 1 percent in its 2008 Open Budget Index (OBI). This score leaves Saudi Arabia ranked 79 out of 85 countries surveyed.
There is basically no accounting or public recording of how the money is spent.
I don't consider this a valid test case for ramp-up stimulus in general, in part for lack of transparency and also because the Saudi economy is so closely tied to the price of oil. Nonetheless it will be interesting to see how this develops.