A tester at Tom's Hardware has just published the results of a huge 85 product test of thermal compounds to measure the impact they have on cooling performance and usability. Liquid metal was found to be the most efficient, but there was a smaller difference across the board than we would have expected.
https://www.notebookcheck.net/Liquid-metal-found-to-be-the-most-effective-in-a-massive-thermal-comound-test-by-Tom-s-Hardware.240236.0.html