Of the choices readily and economically available to painters these days, my personal preference is good quality, plywood panels.  They are heavier than Masonite but in the long run, more absorbent.  They do not warp, and the fine crackles that can occur in the gesso from slight wood expansion can be avoided either by obtaining a high quality piece of plywood from a cabinet maker or by gluing a thin layer of fine linen fabric to the board before the first coat of gesso.