MS5 may be my favorite, but X is probably the perfection of the formula. The removal of slowdown makes the level design of this game shine like no other. It must be the most enjoyable to pick up and replay multiple times, and it's where I think the difficulty has the most positive balance overall. Maybe it has the definitive selection of stages in the series too. Too bad that here the zombie form still doesn't vomit that insane napalm that annihilates the entire screen.