Pulp Alley's pretty good for heavily asymmetrical games. I looked into this recently for a sci-fi game, and it seemed to me to be the ruleset that held up best for very small groups against very large groups. You could easily have a game with, say, two players with five or six heroes each (or fewer) and two others with up to 50 zombies and a zombie leader/necromancer each - so as many as 100-odd zombies on the table versus 10 or fewer survivors.
It's also eminently manageable, because the zombies will all be in gangs/mobs of five, with each figure essentially just a hit point for that group. So running 30 or 50 zombies isn't any harder than running six or ten normal characters. Pulp Alley leagues get 10 slots, which you can fill with five-strong gangs (at 2 slots each) or mobs (at 1 slot each) instead of sidekicks, allies and followers.
There's also a special horror deck for Pulp Alley. I've never used it (can't remember if I have it in PDF). The main rules have provision for bystanders, which could add a lot of character to a zombie game.