depends on how much money you wish to spend. The typical silicon heatsink compound is a metal (zinc) oxide in a silicon suspension. There's premium heatsink compound that contains silver oxides. Those usually offer the best performance which equates to perhaps a 1° to 1.5°C reduction in CPU temps over the conventional type of heatsink compound. Note that some silver oxide heat sink compound conduct electricity and some don't. The latter being a better choice. Remember that no heatsink compound conducts heat like the metal of your heatsink. Use them sparingly.
-=S_Klass=- I tweak... therefore I am.