For an Air-Cooled system it would have to be the (1) Alpha PAL 8045 with a Delta Fan.http://www.micforg.co.jp/c_pal8045e.html
Test results from manufacturer:http://www.micforg.co.jp/c_pal8045e_other_fan.html
Outside test results:http://www.anandtech.com/showdoc.html?i=1532&p=2
Second would be the Kanie Hedgehog W.(Wing)http://www.est.hi-ho.ne.jp/kanie/eproductsbodysocket.htm
See independent test results above.
Third wouyld be the Thermalright SK6. This one is the cheapest yet with the best results.http://www.thermalright.com/sk6.htm
Also: see independent results above.
So there you have it in order.
1-Alpha ---------- PAL 8045
2-Kanie ---------- Hedgehog W
3-Thermalright --- SK6
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