Material Thermal conductivity [W·m−1·K−1] Notes
Diamond 1,000[4]
Copper, pure 401[4][7][8] For main article, see: Copper in heat exchangers.
Aluminium 237[6]
Thinner the better. If you had a slab of diamond, it's 5 times better heat transfer than aluminium.
I remember the days of lapping a heatsink to get better heat transfers. Those days were fun.
You could also try polishing both sides of your IHS.