The perfect game for me is Zombie Cinema. Zombie Cinema is a GM-less rpg about a zombie apocalypse. Each player gets three cards that describe their character. Then the group starts to narrate what happens in the world. The game comes with a board that has a track describing the level of zombies in the game. At first they start out as rumors or items on the news, but as the game progresses the zombies become more numerous and can even gain new powers.
The reason Zombie Cinema is the perfect game for me is that it can be run with no prep. This is a game that can be taken out, explained in 10 minutes, and then played for an entertaining 3-4 hours. It is short, simple, and captures the feel of a zombie movie. The only time dice are rolled is when characters have a conflict with each other, so you really feel the stress of the zombie outbreak destroying the fabric of civilization. It is also inexpensive, portable, comes packaged in a VHS case, and is a lot of fun. If you live in the US Indiepressrevolution has copies for sale.