The worst CEO that I can think of is that of Nokia. In my view, Nokia had the capability of growing a lot and had the power along with the name that which any good smartphone company needs so as to take over the whole race of smartphones.
However, Nokia was pretty laid back in terms of the development of the app store and the capabilities of its smartphones. This resulted in its windows phones to remain unwanted primarily because of the lack of features and the very limited choice of applications that it offered. This is why I think that the CEO of Nokia would be the worst.