The End is Nova
This one is too easy. Super-nova.
No contest! Yeah, humanity will die out as a consequence of its own hubris, zombies, or some other mass extinction event, and if you ask me, specularion on that little detail is pointless. Y'all ain't the be all or the end all sweety. But the world, this planet, the big blue wet thing hutling through a comsic system of proportions you can scarcely imagine; that baby is going out with a bang.
Sort of.
Sound doesn't travel in the great vacum so... One day, Sol (aka the Sun) is going say, "Nope," and then it will silently explode like an emotionally constipated adolescent in the woods. Poof. Stardust and molecules for the solar system. But don't worry, life will go on elsewhere in the Universe until it collapses under the presure of is own expansion.
Don't ask me what happens next. What are you, three?
No, I'm not an optimist, a nihilist, or a pessimist. I'm so much worse.
I'm an alien sent to spy on your species through your internet! Oh yeah, that is probably how you guys are going to die out by the way. I get the feeling that the rest of my people will NOT like you guys. Sorry, but you have no chance of withstanding us with our technological advantage. I guess you weren't expecting a legitimate answer to that question. Oh well. Prepare for a glorious death! For what it is worth, I will miss your cat videos.
See ya, humies.