Lower resistance, higher conductivity.
1 W/m/K is pretty typical for such materials, with much better (10+) available if you're willing to also pay for it.
To do it for any device, use:
Rth = thickness / sigma / area
Thickness is the compressed figure (the gooier materials compress and squeeze a lot, which BTW, beware of that when it comes to screw torque), and area is the area of the device backside (not the full area of the insulator, unless you're using a heat spreader to cover it too). Sigma is the conductivity.
If you're using units of whatever, just plug this into Google and it'll convert everything for you.
The technically-correct answer to your question is the solid pyrolytic graphite, or copper or aluminum heat spreader, but I'm guessing you actually want an electrical insulator, too.
Tim