best one is Gentle Typhoon if you can find them, always sold out because they are so good
i just bought the grey metallic Arctic F12 Pwm Pro(same brand as knc uses)
This one
http://www.newegg.com/Product/Product.aspx?Item=N82E16835186033.
if you do a push pull configuration(which i recommend) make sure you put rings on the one that pulls otherswise it will hit the screws of the Cooling rib.