THE ALIENS ARE ALREADY HERE!! STOP WATCHING THE SKIES THATS HOW THEY RAPE U IN THE BUTT!!!
but for serious. this world will never end....its the people on the surface that are screwed.
Nuclear war? governments aren't going to waste their precious nukes on civilians. each one will target a critical area to try and cripple the enemy. even if whole nations disappear there will always be far of islands and isolated villages that will remain uneffected for a time. and even if this world does become barren eventually it'll heal(40,000years give or take) and keep spinning, and its gonna keep on spinning for a long long time.
Bioterrorism? all forms of terrorism are just a form of "hyper-communication". when a government isn't listening just drive a couple of planes into some buildings, that'll get their attention
Nanotechnology? serious doubts. making a working group for starters is near impossible, even a single error will replicate itself over and over just like a cancer, except being machines they will eventually replicate to a form that has no proper function. then all you need is to play the waiting game.
Epidemic? Humans will eventually grow immune or make a cure. NO MATTER WHAT. "but what about aids?". good question! did you know there are currently 20 people in this world that are recorded to be completely immune to aids. sure its only 20 people but from their gene's a vacanine can start to be made.
Famine? more than 100,000 tons of food a day is wasted all so corporations can make a quick profit.
Anarchy? the only thing anarchy will end is government bigotry and elitism.
Meteor? not in my life time
Aliens (cheesy i know but some people believe): pfft besides nanobots this is the only thing i can see ending us in one generation.