After EXTENSIVE testing for a military application that dissipates over 2500W (Yes, 2.5KW) over an area smaller than 25 sq. Inches, I can say that the absolute best thermal material is Parker Chomerics T777http://www.chomerics.com/products/THERMFLOW_T777.htm
At that website, you can even request a sample. The sample is a sheet about 6 inches x 6 inches, and you can easily cut it up with scissors before removing the backing material.
We've used hundreds of square feet of the stuff, and for a gap-pad, it really is incredible. Without going to screen printed thermal paste or gel, you simply can't do any better. Hand applied thermal paste/gel doesn't even come close - it can not be applied thin or evenly enough to compete.
P.S. I have nothing to do with Parker/Chomerics other than being a satisfied customer.