Zombie: You can deal with zombies, even in a level four outbreak scenario civilization will exist in some shape or another, drastically reduced human populations will exist off of subsistence farming, governments, with their elite troops and equipment, will carve out safe zones free of the undead, and slowly their conquests will expand. Very few cities will thrive in this era, those that manage to be purged will quickly be repopulated, and soon become the walled fortresses of civilization, slowly the purge will begin or the undead themselves will fade away and civilization will rebuild.
Nuclear: Zero survivability period, anyone who doesn't die of radiation poisoning or the blasts themselves will find themselves in a world with irradiated infertile soil, fertility rates will plummet from radiation-induced sterility, any and every piece of fertile land that survives will be fought over like scraps of meat by animals and the conventional weapons of war that found themselves unused in the five-minute conflict will be used by whatever's left of the world's governments. Humanity dies slowly and by the numbers, let's not and say we did.
Disaster: World and human society as we know it is destroyed in a global cataclysm, terrible survivability rate. If the developed world survives it will barely be able to pick up the pieces, the developing world will descend into complete and total savagery from all the devastation, all the up-and-coming nations of the world will be cut down before they had a chance to grow into something powerful.
Alien: Enemy more technologically advanced and/or powerful than us (if you were to go for non-advanced entities like xenomorphs) with either unbeatable technological power or high rate of infection (god help us all if it's the Thing from John Carpenter's film). Low survivability, do not want.