Epic world war — may kill most of us, but a few tribes here and there will survive and humanity will gradually rebuild.
Global warming — may cause a lot of destruction to the earth, probably will kill most animals that can’t adapt, but humans and plants are very adaptable and will survive.
Comet or meteor — possible, though Bruce Willis, Ben Affleck and Billy Bob Thornton would find a way to save us. I highly doubt that humanity wouldn’t find a way out of a real threat.
Sun dying (the movie Sunshine) — by the time this happens, we would already have populated other solar systems.
Rise of the robots/nanos (iRobot, Matrix, Terminator, etc.) — Doubtful, since AI creators would be smart enough to program something in which would prevent that from happening … or the military would be advanced enough to destroy it.
Muslims declare Jihad on the entire world and take everything over — not likely since in general they fight like cowards and most of them still throw stones
Epic space battle with aliens (the book Ender’s Game) — Doubtful we would be killed completely. More likely we would be colonized.
Supercollider experiment gone wrong (i.e. black hole) — highly doubtful, but minutely possible
Plague that kills everybody (30 days later, I am Legend, etc.) — the world is too vast for this kind of thing to reach everywhere, and viruses need hosts to live in, so it would die before reaching everyone. Or, it would cause people to mutate, so there’d still be some remnant of humanity.
Honestly I can’t imagine a scenario where humanity kicks the bucket completely. Do you have any good ideas?