People who know me probably think of me as a pessimist, but I don't really see society ending itself. There's too many people and something would kick in. Large scale problems or conflict, sure. But I think society will end with an astrophysical event before anything self-inflicted.

If I were forced to choose though, I'd probably say nuclear warfare. It's just too powerful and will eventually proliferate and there's too many psychosocial risks behind their use.