The best magnetostrictive material for energy harvesting applications is not necessarily the material with the highest magnetostriction strain. In this paper, based on the description of the Ericsson cycle, a simple criterion to define the most efficient material is proposed. The criterion takes into account the accessible range of stress and magnetic field. It relies on four material parameters only, namely the initial magnetic susceptibility, the saturation magnetisation, the maximum magnetostriction strain and the coercive field of the considered magnetostrictive material. Mass density and price are also involved if a weight optimisation or a cost optimisation is sought. The potential of several materials is compared based on this approach, and it is shown that Giant Magnetostrictive Materials are not systematically the best choice for energy harvesting applications, challenged for some operating conditions by electrical steels.