Stephen Hawking believes that mankind's biggest threat is from Artificial Intelligence, which isnt an option on the thread.
It'll probably be from one of the following:
- Plague/Epidemic
- Artificial Intelligence (on some Terminator movies type shit)
- Religious Upheaval (think Book of Revelations)
- Overpopulation (too many people polluting the Earth before we have the technology to make Mars inhabitable)
- Nuclear War