The National Astronomical Observatory of Japan is number one. With the latest release of The Green500 List, accelerator-based supercomputers now occupy the top eight slots of the Green500, where the 'fuel efficiency' (or energy efficiency) of supercomputers is defined as 'millions of floating-point operations per second' (MFLOPS) divided by 'watts' (W) or MFLOPS/W. Accelerators refer to the use of dedicated hardware to perform computations faster than a traditional processor, also known as a central processing unit (CPU).
See the full article at http://www.green500.org/lists/2010/06/little/list.php. Now someone please fix the spaghetti.