Well, drat. Always a wrench in the works....
If you HAVE to do a frontal shot, you have a problem. The skull is thickest in the front. If you have to take a shot through the skull, then I would opt for penetration as opposed to expansion.
For most instances, I'd take a look at a high speed full patch (jacketed) bullet. A 9mm Parabellum--probably a 124 grain or better--would get the nod. Barring that, a .45 ACP loaded to standard ball ammo specs (850 fps or better) will do; so will a .357 with full jacketed loads--or even something shooting a HARD cast bullet.