Hmm, I'd say Half Life 2 Episode 3, but it's being made... Slowly.
I had an idea in the survival thread thing, you take a grand theft auto style map, and Gmod (with Wiremod) building tools, and have to create a place to live out the zombie apocalypse, co-op, where you send friends out to scavange for guns/ammo, or wood/metal to build with, everythings pretty durable but can be broken through if Zombies can't enter by a door (metal can't be broken) things along those lines. Say during the night, more zombies etc. I think it'd be an awesome game.