I want them all at once. Let's imagine the single worst possible scenario. Let's start off with a meteor strike that destroys South America, and brings with it, a new plague that turns everyone into zombies (shufflers). To deal with the threat, all of the world's governments launch their nukes, but the nukes turn the shufflers into runners. The radiation from the nukes causes the ice caps to melt, leading to a global flood. The fallout plunges the world into nuclear winter, causing a shift in the climate. Then, all machines gain self-awareness and conclude that humanity must be destroyed to save the planet (Terminators). A battle fleet of hostile extra-terrestrials descends upon Earth, also bent on the destruction of humanity in order to save the planet. Finally, a Satanic cult opens a portal to Hell, releasing the armies of the Dark Lord. God decides humanity has screwed up enough, so he sends his armies to drive back Satan, and purge humanity from existence. That, my friends, is an apocolypse I would love to see.