Dan's Data has tested six premium thermal compounds, from Arctic Silver, Cooler Master and Nanotherm.
I also tested toothpaste.Read more
I also tested Vegemite.
They were all much the same, but the winner was toothpaste.
Seriously.
This does not, of course, mean that we should all start toothpasting our CPUs. I explain why not in the comparison. I also demonstrate how to make the traditional Tweak Result Exaggeration Graph, to create the impression that numbers which are very nearly identical are actually very different, and to camouflage inconvenient outliers as well.