The opening of tv shows are kind of like TV adverts in which they deliberately try to use images and sounds (particularly theme tunes) that stick in your mind, since you will be watching it over and over.
What are the most memorable for you? The show doesn't need to be good, we are specifically talking about the intro.
e.g. this show was shit, but had an awesome opening
For me it would probably be Quantum Leap, aside from being one of the best sci-fi shows made, it had an excellent intro with sexy concerned lady.