Hmm, the program has to be long running and has to either appeal to a wide audience, or gather a cult following from a niche audience. I am going to cheat and nominate two programmes, for first and second place.
1) Doctor Who. This speculative programme ran for many years and attracted Sci-fi fans, thinkers and the youth audience. I am a fan of the earlier Doctors, but the recent Doctors have gathered a whole new generation of fans. The new series have broken the original formulae, but it goes to show the power of this concept that the show has evolved and captured more fans.
2) Buffy the Vampire Slayer. A combination of Gothic and Soap Opera elements. Very character driven, attracted a loyal following of both young people and adults. Very watchable and re-watchable.