The most energy efficient CPU cooler is simply a huge heatsink (no fan). The case must be large enough for you to install it though.
For example, the C2D E6600 (65w TDP) in my HTPC is passively cooled by a Tuniq Tower 120. It runs at stock speed and it does not get too warm when stressed (i.e playing games), I think 68C - 72C. But it is really only used to watch movies so actual temps are less than that. I also have a passively cooled nvidia GT 9600 in the HTPC.
The Tuniq Tower 120 is about 150mm tall, 130mm wide, and 110mm deep. That translates to about 5.9" tall, 5.1" wide and 4.3" deep. It weighs in at 800g or roughly 1.75lbs = 1lbs 12oz. Heavy enough to use as a weapon....
I already had that heatsink anyway so I simply reused it. It was my for my gaming rig before I built a new gaming rig around the Q9450 (also using a Tuniq Tower 120). Since the E6600 was simply running at stock speed and the case has good enough air flow, I simply decided to remove the fan. While 68c - 72c is warm, it is not considered hot.
As for a low speed fan vs. a massive heatsink... well you can't get more energy efficient than having a cooler that does not use any electricity. However, passive cooling with a massive heatsink generally works only with computer cases large enough to accommodate something that large.