Nuclear war is the only real option because it offers the greatest chance of survival.
Comet/asteroid = mass deaths, planetary-wide climate change, no sunlight (no plants, no plant eaters, no eaters of plant eaters) and millions of years before recovery really kicks in.
Zombies, of any time, is a certain death knoll, because ammunition eventually runs out and everyone will make a stupid mistake, get bitten, and become a zombie. That's why there's so many zombie movie sequels.
Aliens will simply not be friendly. They will be looking for resources and we will be in the way and eliminated through some magic-like technology that renders us vapor instantly, across the entire planet simultaneously.
Any other plague is maybe survivable, but if it's a plague we haven't seen yet, then it's likely to be very drug resistant. And the only people who will be immune are the oligarchical political leaders that hide in bunkers until it's over, and I don't want to be around when they emerge.
So the only real survivable option is nuclear war. Modern nuclear weapons are quite efficient and deliver most of their damage as heat and shock waves, rather than radiation and eventual nuclear fallout. I live in the country so am unlikely to be a direct target. Less mass extinctions than with an asteroid strike, and the oceans are likely to be mostly unchanged except for radioactive runoff.
All in all, I think Nuclear Armageddon offers the greatest chance of survival for the greatest number of people, leaving the greatest amount of human infrastructure and planetary species intact.