There is no single "best". There are several that are constantly at the top of the list, but there are to many factors to have 1.
OCZ Freeze seems to be the new flavor around here.
MX-2, IC7 Diamond, TX2, AS 5 are also all good choices. There are others also.
Correct application, airflow and cooler used are as important as which of the top TIM's you use.