An Unearthly Child
The first Doctor Who serial. The first episode is one of the finest television programmes ever made, but after that... well, it's just a huge pile of tedious boring old cack, isn't it? It takes a special kind of incompetence to come up with such a great idea for a series, fill it with great regular characters, and then manage to bore the audience half to death.
Not an original opinion perhaps, but nonetheless the CORRECT one.