No, really.
  1. 26.
    Update: Moved item up three spots from No. 7 to No. 4, in light of current events.
    (Didn't think this list would need updating... ¯\_(ツ)_/¯ )
  2. 25.
    The Rapture
    Not going to happen.
  3. 24.
    Death Star style annihilation
  4. 23.
    Alien Invasion
    They're out there... but are too far away to get here.
  5. 22.
    Big Crunch
    One theory. Literal doomsday. Very long time away.
  6. 21.
    Sun Goes Red Giant
    In 5 billion years, if we survive the rest of this list. 😬
  7. 20.
    Space-based plague
    Comet or asteroid or returning spaceship borne plague. Far-fetched.
  8. 19.
    AI Singularity
    Could happen? And if it does... could be bad for humanity?
  9. 18.
    Vectoring Fungal Disease
    Apparently we don't know a lot yet about amphibian fungal diseases...
  10. 17.
    Space Colony Uprising
    Taxation without representation!
  11. 16.
    Robot Revolution
    More likely!
  12. 15.
    Massive Solar Storm
    Takes down our electrical and communications grid. Not if, but when.
  13. 14.
    Engineered Disease
    Escapes the lab, kills us all
  14. 13.
    Genetically modified superhumans
    They are already among us. How long until the transhumans no longer tolerate us mere mortals? Or out-compete us (see also: Neanderthals).
  15. 12.
    Melting Ice Caps Release Ancient virus
    To which we have no immunity.
  16. 11.
    Leads to resource depletion
  17. 10.
    List App update removes comments
    Aaaaaaahhhhhhhhh! Chaos!
  18. 9.
    Asteroid direct hit
    Remember the dinosaurs?
  19. 8.
    Weaponized Biological Warfare
  20. 7.
    Super Volcano Eruption
    Similar result as asteroid or nuclear winter. See overdue Yellowstone Caldera.
  21. 6.
    Trans-species Plague
    Ebola? Zika? HIV? Bird flu?
  22. 5.
    Tribalism/Religious/Civil War
    A Tale as Old as Organized Religion ... leads to WW3 and/or WW4, and/or etc
  23. 4.
    Hubris / karma
    Bumbling Fools with nuclear codes.
  24. 3.
    Global Warming
    Leads to extreme resource depletion and turf wars.
  25. 2.
    Extreme Resource Depletion
    Let's fight for fresh water, food, oil, minerals, labor, energy, etc etc etc
  26. 1.
    Thermonuclear War