No house rules, play as the Great Frog Intended. Many years have been spent balancing all factors of this game by the lovely folks at FFP. After lots of games it will start to balance out. I do believe every scenario is winable for the Heroes. I also believe the odds are stacked in the Zombie's favour, yet I quite like this. The Heroes should be on the edge of their seat, each game, tense and every victory should be hard fought.
A few students and teachers etc. vs. endless hordes. What odds would you expect?
The exploration markers are what make the game really interesting for me: not knowing if it's going to be a shotgun, another hero, or a zombie waiting for you in the next building. Escape in the Truck is what I always play with people new to the game, with exploration markers, heroes replenish, and grave zombies (to make it more exciting for me as the zombie player).
After foolishly playing it back to back three times with a fellow masochist, I'd finally had enough of it and threw the Die Zombies Die scenario card into a fire. Okay, I'm exaggerating, but it's stayed at the bottom of the box and has never been pulled out again.
Burn it to the Ground I've only played once as the humans. I gave Billy the collected three tanks of gas, and I kept waiting for the fourth one to emerge. Finally I got a "Just What I Needed" card and went searching through the deck for it, to then realize there were only three gasoline cards. Whoops.
We're still playing through the other scenarios and have yet to find a favorite. I suspect it will probably be one of the more complicated ones available for free download on Flying Frog's website. "All Hallows Eve 2: The Ritual" sounds particularly exciting.
Zombie Apocolypse with the Zombie Horde is worth a go.
Mountain of the Dead is a fun one