Global Warming.... by the time it actually goes from being "it could be a threat" to "GOD DAMN IT'S KILLING ME!!!" we'd have found a way to prevent it would be able to agree on wether or not it even exists... (my oppinion is not included on if it does or doesn't exist).
Space Disaster.... anything could happen. But when was the last space disaster? Many people believe that asteroids and meteors could hit the earth and wipe out entire species of people. What people forget is that the earth has gotten smacked or at least pinged (barely hit) by an estimated "hundreds of thousands" of "doomsday" missiles. We've pretty much, along with other planets in our system, have gotten hit by all the locals floating around that could pose a threat. The chance of getting cought by something free roaming not of our system is low - but could happen. Then again we survived ice ages before. And we have wool now.
Stars exploding, and all the odd energies out there... I just don't see an implosion happening any time soon. If we aren't dead by then, then that could definatly put a frown on things.
Mankind... we'll... the only way I can see that honestly happening is full on nuclear war... of course theres a nuclear scare among people thats pretty much worked thus far... if we bomb them they'll bomb us kind of fear. May not always hold out, but that's the only way I can image this. And even then I can't see it wiping out the entire planet, including after affects.
Robots... I'm not even going to comment on robots.
Bacteria, viruses, etc... The flu has killed more people than any single thing in the entire human excistence. It's plagued us for as long as theres been an "us". We have no cure, but precautions and some treatments. HIV/AIDs is bad, and for it's "youth" compared to others it's done some nice damage. And while I'm not sure if a cure can be found, the sickness can certainly be (nearly)eradicated if safe sex and other precautions (blood safty) were made. We're a pretty tough species given our ability to adapt and survive. This, out of the options, is the best choice I'd say.
However, my actual thoughts on extinction isn't on the list. While I don't think the rapture will happen, AtM's got a good point on the few options on the list here - not even an "other". Me, I'd have to say malnutrition.
Now true, malnutrition can be caused as an after effect of many of the things listed - but that's my choice. Given that malnutrition is the 2nd biggest killer of humans in history - far out doing temperature, murder, cancer, everything you can think of except the flu is way behind malnutrition, it just makes sence to me.
So that's my pick.
EDIT:
Actually I don't think that mankind will be wiped out by any single thing. Like most extinctions I'm sure our populatio nwill dwindle down little by little, as a result of multiple actions. And I'm fairly sure that there will be a last group of humanity. And some will probably be sick. But I don't think the sickness, be it the flu, aids, radiation poisoning, or whatnot will be what kills them. They will probably starve, maybe have such a poor diet, or simply not have enough water. The last human, in my eyes, will die of malnutrition.