Plague. Nuclear war. Mayan calendar. Global warming. Aliens. Zombies.
Once you've picked that, you must lay out a world history so it all makes sense.
Then some brainiac says: "That's not plausible for this reason!"
The whole thing falls apart because you're not a real cultural anthropologist and physicist and historian.
But you know what works? Use a narrator who doesn't know any better and just reports without bias the bits and pieces of shared stories from the survivors. Still need a plausible disaster, but you let the reader fill in the blanks his or her own way.
Oh, I know what happened, but I'm only leaking it in little drips and splatters.
I use lots of campfire chat. I also believe when a global cataclysm strikes, no one will know the whole story. Most people will apply their limited knowledge and experience to lots of paranoia and fear, ending up with a mostly false but very satisfying story.
Back to my feelings on omniscient third-person narration: novels that begin with an in-depth explanation of everything that happened across the world that lead us to this cataclysm strike me as unnatural.
Plus, it's hard.
“Always ecstatic or angered. One day
Gives lobster and champagne to everyone
Saying: ‘This is the end. Lets go out strong.’
Next day sells K rations, antivirals,
And wind-up radios out the back door.
Meanwhile, I try to run a restaurant.
Crazy don’t make good hospitality.
Needed some mountain air, get fat on trout.
Changed my voicemail greeting to: ‘I quit, Tweak.’
Took my ass and his best whiskey hiking.”
From Cathartes Aura on the Road from Nowhere, Chapter Seven, Verse Ten.