I’m torn between suggesting something everybody can find interesting and something that may be an overlooked classic. But I’m going to go with “The X-Files”, particularly as they have helpfully separated the “conspiracy” and “monster” episodes in the DVD releases.
IMHO, “The X-Files” was the smartest, scariest, sexiest show on television in the 1990s. “Fringe” and “Eleventh Hour” and “Lost” are just inferior copies, who don’t have a fraction of “The X-Files’” depth. “Fringe”, in particular, completely lacks the tension between mysticism and skepticism, and instead throws any half-assed pseudoscience at the viewer uncritically.
I don’t know if the show has aged terribly well. It seemed to belong in that period between the fall of the Soviet Union and 9/11, with the US had no real external enemies but that Cold War paranoia had seeped into the culture at every level. Nowadays, there is so many bizarre diseases and government dirty deals and weird natural phenomenon going on that “The X-Files” starts to look quaint.
Also, we’re facing a glut of shows with ongoing plotlines that concern gradually revealed information about complex backstories. When they’re done right, they’re epic storytelling. When they’re done poorly, who cares what dark Gothic secret is haunting the protagonist?
Although “Twin Peaks” is probably the true progenitor of long form, gradually-revealed-mystery television, “The X-Files” is the most successful example.