A question like that is like a which is the best car question.

You're going to get so many opinions that I might as well throw in mine.
VANTEC AeroFlow VA4-C7040. For AMD socket A 462 and Intel 370/FC-PGA/FC-PGA2. Retail. Vantec Vertical Aluminum Fins with Copper Core Heat sink + T.M.D. fan, and triple clip for secure instalation.
Specifications:
Supported Socket type: Socket A/462 and Socket 370
Material: Aluminum with copper
Fan Speed: 5600 RPM
Air Flow: 35.5 CFM
Noise Level: 38.0 DBA
Bearing type: T.M.D.
Dimensions 74.5x74.5x60 (mm)
Life Cycle: 80,000 Hrs. (Comes with thermal Compound)
Notice it has the TMD fan. These fans offer a great combo of airflow and low noise. Try to avoid any fan that has much higher DBA than this one. Also take a look at
Rob Rich's news article from 6/29 about the all copper SLK-700 with the TMD fan.
http://www.techimo.com/newsapp/index.pl?photo=5302
Mike