>>35960833Z-moves. While Mega-Evolutions are cool, they are hyper-specific to certain mons and they are set in stone, stat, type and ability-wise. They are inflexible and mostly have already been power-crept out save for a few good ones like Sciz and Venu, and even then, they aren't what they once were in ORAS.
Z-moves, on the other hand, give a fresh and unpredictable boost to literally every Pokemon. Even the most dogshit mon can have a nifty gimmick that can completely surprise your opponent and swing games. Z-moves may not be any fun in single player but in competitive pokemon, they are much more welcome than mega-evolutions and will have greater staying power as the series goes forward.