Delphox and Greninja are the only 2 starters I've beaten all 8 gyms with. Delphox has a brilliant Special Attack, and an early Psyshock (Lv 33) and Flamethrower (42 or something). Access as well to Grass Knot + Shadow Ball relatively early is nice as well. Greninja can wreck early on with Water Pulse + Round, with Rock Smash/Thief/Return/Quick Attack/Rock Tomb as coverage. Extrasensory, Surf, Night Slash and Ice Beam/Acrobatics will serve you well. Mine knows Waterfall until I get to the Pokémon League.