My favorite would be zombies. Realistically it's extraordinarily unlikely, at least in terms of involving walking dead people versus a more realistic plague approach with live zombies, such as in "28 Days Later".
The most likely EotW scenario . . . hmmm. I could see nuclear terrorism occurring, but while horrific, that wouldn't be the end of the world. It might trigger an economic collapse of the world economy, resulting in many of the world's nations dissolving into chaos, but again, that's not the end of the world and a hundred or two hundred years later, things would probably be more or less back to normal.
Of course if that nuclear terrorism somehow resulted in the use of the nuclear arsenals of China, the U.S., and Russia, that could indeed be an EotW scenario. I could see something like this happening. Pakistan's government collapses in a take over by a radical Muslim religious coup. The new rulers had men on the inside in the military and are able to acquire the codes needed to use Pakistan's nukes. They use these nukes on the U.S. I don't think they have ICBM's, but maybe they put them on cargo ships and oil tankers and explode them in harbors of our major cities. Russia and/or China warn us that if the U.S. nukes Pakistan in response, that they will nuke us right back (because of Pakistan being uncomfortably close to those countries). The U.S. ignores the warning and nukes Pakistan anyhow and WWIII breaks out.
While an asteroid or comet or super volcano could certainly do the job, it could well be millions of years before such an event, so I think a human-triggered apocalypse is more likely.
Incidentally, the word "apocalypse" literally means "revelation", as in, "I've been trying to think of a solution for days and I just had an apocalypse!" The last book in the New Testatment of the Bible is typically called "Revelation", which is short for "The Revelation of Saint John the Divine". It refers to a vision that John had in which many things were revealed (hence the term "revelation"). It was once commonly called "The Apocalypse of Saint John the Divine", which means exactly the same thing. As the book of Revelation is full of horrific end of the world events in which (if memory serves me correctly), roughly two-thirds of the world's population dies and the rest are going to die if not for divine intervention, the word "apocalypse" acquired a different meaning, based on the events described in the book as opposed to the origin of the book.
The most easily triggered world wide EotW scenario would be a biological super weapon. It would have to have the following characteristics.
Highly contagious - airborne and/or transmitted through a very common vector like rodents or mosquitoes. The vector should either be immune to the pathogen or able to live long enough to reproduce in large numbers before the pathogen finally kills it. Ideally even if there are no human hosts around to keep the pathogen going, the vector would keep it going to pick off any human survivors that stick their heads out of their bomb shelter 20 years down the road.
Able to mutate rapidly just enough to keep old vaccines from working (like the flu).
Inside you forever once you are infected (like herpes or HIV).
Very high mortality rate, or possibly a sterilizing effect. A sterilizing effect would ultimately have the same effect on the human race as a lethal effect and might be detected a lot later, especially as children and couples that don't won't children yet would not realize that they had been sterilized.
Long period of time in which there are no symptoms (like HIV). If you can be infected for a few years while you spread the disease to everyone you contact without ever realizing you are infected, that would be rather nasty. It would also insure that there would be no place on the globe that was not infected as you would have a global version of "six degrees of separation". For example, I am infected and I go to church with a guy whose brother is a missionary in India. The missionary in India is a medical missionary who infects thousands of patients. One of the patients has a cousin who is a diplomat to the U.N. The U.N. diplomat infects all the other U.N. diplomats, who in turn infect all the world's leaders. Okay, maybe that was seven degrees of separation. I lost count. But you get the idea. Nobody is safe.
Where would such a disease come from? After the collapse of the USSR, much of their weapons technology was insecure. Someone may have acquired such a biological weapon and simply be waiting for the right buyer.
There could be someone who is waiting until they develop a vaccine before releasing the disease. They might envision vaccinating their fellow countrymen or members of their movement so as to take over the world after everyone else is dead. If the disease mutates afterwards so that the vaccine no longer works, that's just tough for them.