1. Nuclear War.
2. Super Plague.
3. Biblical Apocalypse.
4. Zombie uprising.
5. Alien Invasion.
6. Earth pushed out of orbit.
7. The return of the Old Ones (Cthulhu)
8. Computer uprising.
9. Black Hole.
10. Negative strangelet.
11. Grey goo.
12. The Sun undergoes rapid expansion.
13. Verneshot. Earth explosion.
14. Reality takes a holiday.
I can't pick just one!
Aliens invade Earth, and upon them not being too receptive born again Christian evangelism and subsequently being told that they will burn in hell, the aliens decide that humans aren't friendly after all and decide to release a Super Plague tailor-made to eradicate humans from the planet. Once this is discovered, religious persons high up in the chain of command view this as a Biblical apocalypse and the aliens as demons come from the underworld of outer space. Nuclear missiles are launched, but the aliens deflect most of them, so that the warheads detonate back on Earth. Since the flying saucers were hovering over only one side of the planet (not the Pacific ocean) the nuclear payload has ever so slightly nudged planet Earth out of orbit, but most people are too worried about the fallout and the plague to take notice.
This destruction accelerates the bunker mentality of mankind's military forces. They build and network massive supercomputers to find a weakness in the aliens' defenses. The result is a breakthrough in nanotechnology and the military launches grey goo onto the alien ships. Only a very small amount drips down onto the mainland. Also the supercomputers become sentient and start manufacturing killer robots. No one really notices because the military budget is classified.
Thoroughly perturbed that their interstellar luxury liner ships are slowly being molecularly disassembled, the aliens retaliate by firing their Solar Chrono-accelerator Ray at our sun. As they leave for the nearest spaceship mechanic shop in the quadrant, they mouth obscenities and find some smug satisfaction that Sol will rapidly (at least in the next few years) grow to 250 times its current size, engulfing Mercury, Venus, and Earth (if it is anywhere near its current orbit). Fortunately the sun will afterwards contract an form a black hole.
But this doesn't matter really because most of mankind is battling killer robots in a wasteland infested with fallout, plague, and grey goo. And because Haiti and New Orleans feel left out of these latest disasters (and Jamaica doesn't anticipate having another bobsled team), the Union of Orthodox Voodoo Priest decide to cast a powerful hex to raise to the dead.
With all of the commotion occurring, people seem to forget how much more zombie, robot, and grey goo trash is getting tossed into the rivers and flowing into the oceans. And this is really pissing off the Old Ones. Cthulhu returns and gathers a massive army of squids, octopi, and humans with aquatic hobbies. This confuses most fundamentalist Christians who were totally convinced that the aliens were the evil demons.
Fortunately the evil supercomputers decide that the world is royally screwed up and decide that they can remedy the situation by building a device that will sent a killer robot back in time to terminate Christian Bale's mother. They are probably right because supercomputers are really smart. Too bad their Temporal Displacement Device (TDD) is actually a Negative Strangelet Generator (NSG). What is left of the Earth is quickly changing through a wacky chain reaction of sub-atomic particles. Cthulhu beats a hasty retreat back to his own dimension, and decides that humanity should receive his parting gift of an exploding Earth.
Suddenly reality finds this current situation far to weird to endure and goes on holiday. And suddenly it's like we're all on LSD except it won't wear off. And since everything was already jacked up, it is a very bad trip.
