1. THE X-FILES (FOX, 1993-2002)
Mystery: What is the government hiding about aliens?
Since the start, The X-Files asked one question: How much does the government really know about alien life? During the course of the show, Agents Mulder (David Duchovny) and Scully (Gillian Anderson) obsessed over a potential government conspiracy involving the existence of extraterrestrials, which all started when Mulder's sister was seemingly abducted when they were children.
The show would give little hints here and there, and it would often half-answer many questions, but ultimately a lot of the big issues were left unresolved. Despite the lingering plot threads, The X-Files was a cultural phenomenon in the '90s for good reason, as fans would tune in by the millions every week in hopes of discovering the truth. This is one of the rare mystery shows that is actually more interesting simply because so many questions are left unanswered by the end.