Aluminum is the most popular material in high thermal conductivity boards.
Normal thermal is from 0.8-2.00 W/m.k,and higher can up to 8.0 W/m.k.
Here is an example of Yamaha power board,this is double sided board with metal core. The Aluminum is in the middle,and copper layer on top and bottom side.From top side,we can see there are TO-252 footprint parts,these components have very high heat when working. so high thermal conductivity board is required .The design is very great as there are many thermal vias at each TO-252 parts.The heat will go through another side soon .