According to DaSickNinja's comprehensive review, Masscool Shin-Etsu x23 is the best you can get. The full review is Here and I doubt you will find a better comparison anywhere on the web. As x23 can be somewhat difficult to find, AS Ceramique is a good alternative.
IC Diamond 7 Carat Thermal Compound if applied right through sheer physics give it the highest ability to pull heat from the cpu and into the heatsink. any of the diamond compounds, and diamond compound also has way higher heat tolerance than and silver/ceramic ect.