The green one (012233 CRAFTFOAM GREEN)
Its pretty stable in my eyes and doesnt flake too much when i'm cutting it. I think I got the 600x600x25m with 12 in the box. lil expensive but i've still loads left after working on a Mordheim board, An arena and a warhammer fantasy castle!!
Simple layer of pva glue and its ready for whatever i need
Forgot to say 2 of the boards by 2 makes almost a perfect 4x4 foot playing field