For fear of reviving an (un)dead post ... you guys have overlooked the possibility of a zombie-like-alien invasion. Think along the lines of "Invasion of the Body Snatchers" ... the Pod People. Those guys didn't have to do much to have us wiped out; we just had to fall asleep and a pod would clone us and we'd be gone. That's a tad more freaky than a Zombie Apocalypse (which arguably is going to be loads of fun; for mostly the wrong reasons) or a big-headed brainiac alien invasion.
Just try to stay awake for more than a week and you'll realise the whole planet is doomed. Zombies are relentless in their slow pursuit of every piece of flesh they can get their hands on but they'll take time to hunt us all down, and in some cases we'll be safe enough hiding away in our bunker in Tibet. Smart aliens will probably also struggle to find every survivor (at least, that's what Hollywood tells us). Pod People on the other hand? They just need to wait for us to fall asleep O_O
