The sun collapsing. Or exploding. Or maybe some sort of massive solar flare that cooks us.

I don't see any of the other stuff mentioned being able to wipe us all out in one 24-hour period. Nukes could do it, but all of our nukes are aimed in roughly the same places. Most of Africa and South America would survive the first wave, no question. It'd take weeks or longer to wipe them all out in the aftermath.

Aliens could do it. A Deathstar-style laser that explodes the planet in a couple seconds? It could happen, I guess.

Supernatural? I'd call that a distant third in likelihood. Most supernatural events can be countered with a quickly spoken latin phrase, or maybe a wooden stake.. if the Winchester's have taught me anything.