I don't know about "best", but I'm running an Athlon64 3000+ with the heat-sink/fan combo that came in the retail box, and I have no complaints. While idle the temps are 38 CPU/38 system, and under full load the temps are 45 CPU/41 system...I've never once seen the CPU temp over 45. I'm using an Antec Sonata case with two 120mm case fans (the fan included with the case as the intake fan, a slightly more powerful ThermalTake fan as the exhaust fan), so I get some pretty good airflow through the case as well, which of course makes things easier for the CPU fan, but like I said, I've got no complaints about it at all.
EDIT: I replaced the thermal compound that comes preapplied to the heat-sink in the retail box with some Arctic Silver 3 I had lying around by the way. I don't know what (if any) impact this may have had on the fan performance.