X-bit Labs went through the trouble of testing 26 different types of thermal greases on an overclocked AMD Radeon HD 6970 graphics card, you can check out the comparison over here. The Nanoxia Nano TF-1000 Fluid Metal scored the best but it's quite expensive and difficult to apply and remove.
Next are the group of high-efficiency thermal interfaces, a total of 12 products receive a Recommended Buy award from X-bit Labs: Thermaltake TG-1, Tuniq TX-3 and TX-4, Thermalright Chill Factor III, Gelid GC-Extreme, Cool Age CA-TC3 Nano, Nanoxia Heat Buster, Arctic MX-3 and MX-2, Zalman ZM-STG2 and GlacialStars IceTherm II. Especially the latter one is quite interesting, because it costs just $5 yet still delivers pretty good results.
Another interesting observation is the massive difference between ThermalTake's TG-1 and the TG-2, the TG-1 is the second-best thermal interface in the test while the TG-2 is the worst performer.