As a reminder, I don't really do horror.
That said if I had to choose one it'd be All Flesh Must Be Eaten.
Having used it for different sorts of games but keeping an undead element I think it works really well in general. The archetypes in the base game + expansions give anyone the opportunity to be a Survivor and it is the way these archetypes are built and the mechanics around them are used that stick with me.
Almost 2 years ago I wrote a post about Character Motivations (jeez, 2 years!?) and that references a scenario I ran using AFMBE. The key thing to remember is that I could have removed the undead element from the scenario and yet I still would have chosen AFMBE as the system as it's gritty and deadly enough to match the scenario itself.
Other Horror games I've played -
- Call of Cthulhu - Not my thing...
- Chill - MANY moons ago and I really can't remember it.
- Savage Worlds - Rippers which is horror genre but not really a scary game to run or play. Feels more "pulp horror" than anything else but then again that's Savage Worlds to a tee.
- Unknown Armies - A game which I could only scratch the surface with and similarly used for an X-Files-esque scenario.