What is the best foam to build a RC plane wing from?

Depends on its plane type and size. Most often its expanded bead foam. Its color is white and comes in different densities. What most R/C planes use is 2lbs per sq foot density white foam. High performance gliders sometimes use the pink or blue foams. They tend to be denser and heavier than the white expanded bead foam. All of the require a foam cutting bow and put off fumes that are in some cases toxic so a mask should be worn while cutting. Precut foam cores for a lot of planes can be bought from Eureka Aircraft Co and what they don't have they can make for you. You need to supply them the length, root, tip, LE set back and airfoil you need plus some other info.

Thanks for the info.