Ecosystem collapse seems unlikely, while people tend to panic, we're smart, we have people with experience in the working class, or we can learn ourselves, we'll be able to come up with (probably a sub-par but still working) economy.
So does AI, people will not just let robots take more and more jobs, there will definitely be protests, and a lot of them.
I voted nuclear war, but I think either that or climate change. As Smug said, while we're smart, we're also selfish, not really caring of the things around us, and willing to do literally anything for personal gain, and thus eventually we're gonna be destroyed by us.