Which is the most sustainable form of rigid insulation?

[DianaW]

I'm having my uninhabitable attic re-insulated and boarded (where there's enough headroom) to store stuff there but the contractors don't do loft hatch insulation. That leaves a big gap in the insulation layer that I need to remedy.
I apparently need to attach rigid insulation to the attic side of the hatch cover but I don't know which form is most sustainable nowadays. Help, please!