Right now, nuclear armageddon, while possible, doesn't seem likely right now.
I think that Yellowstone erupting is inevitable, and will probably kill most of us here, especially in the U.S., as the ash suffocates most of us, and our crops, though as a race, we should recover.
My biggest fear is a genetically modified bioweapon getting out and causing a pandemic. A natural pandemic would kill most of us, but enough should be immune or otherwise survive to rebuild.
Otherwise, the world will probably end when the sun expands, though by then, we'll be long gone, one way or another.
Asteroids and Meteors are the wild cards, and there is no telling what they'd do if we aren't able to do anything about them.
I think there is enough hints that extraterrestrials have been visiting our planet for a long time, and if they wanted to invade, they would have done so by now, and there wouldn't be much we could do to stop them, whether it be enslavement or wiping us out wholesale.