The X-Files, FOX
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Caution: potential spoilers.
One of the best shows of all time (or at least I thought so at the time). It was, IMO, the definitive series of the 90's. (It ran for nine seasons, from 1993-2002. I'm fairly sure I didn't have FOX until 1995, so it's quite possible I missed the first two seasons. I really don't remember; I might have found some other way to watch it. Or not.) My review could just as easily go under various categories, such as "science fiction" or "mysterious" or "weird," but I think generally speaking, "supernatural & paranormal" tends to be the best fit. Um... the show alternated between "monster of the week" stories (which could be about actual monsters, or aliens, or mutants, or psychos, or whatever) and "mythology" stories. (This was the first show to use the term "mythology" to refer to its complex, interconnected, over-arching mysteries and conspiracies. The "mythology" or "myth arc" concept has been used by any number of other shows, since then.) Anyway, throughout most of the series, the main characters were Fox Mulder and Dana Scully, who were FBI special agents. (The stars, David Duchovny and Gillian Anderson, acquired reputations as, like, "the thinking person's" sex symbols.) Mulder worked in a division called the X-Files, and investigated all sorts of paranormal stuff, including aliens and whatnot. Scully was a skeptic, a doctor (pathologist), and a lapsed Catholic. There were people who didn't like some of the stuff Mulder was up to, so they teamed Scully with him, hoping she'd gather evidence to debunk his work. Oh, and I should mention Mulder and Scully's boss was Assistant Director Walter Skinner (Mitch Pileggi). He was more or less on their side, though it wasn't always easy for him to be. So he wasn't always particularly happy with them (especially not Mulder).
But Scully didn't find any evidence against Mulder. Over time, she came to realize most if not all of the out-there stuff he believed was actually true. Some of the show's mysteries involved Mulder's family, going back to like Roswell or something. His father had been involved with the Syndicate (the major conspiracy group of the series), I believe, and when they were kids, Mulder's sister had been abducted, and never returned. Samantha's abduction was a large part of what drove him to seek the truth. Anyway, all kinds of strange things happened with the mythology over the years, a great deal of it related to Cigarette-Smoking Man (William B. Davis), who was the member of the Syndicate most familiar to the audience. (He was known in the fandom by various names, such as "Cancer Man" or just "the Smoking Man," though I recall often seeing him referred to as "CSM," so I always go with "Cigarette-Smoking Man." We did eventually learn his real name, though. Which I won't reveal.)
Anyway, eventually Mulder disappeared (around the end of season 7, I think), and agent John Doggett (Robert Patrick) came in to head up the search for him. And a little later another agent, Monica Reyes (Annabeth Gish), got involved. They were both assigned to the X-Files, and were the main characters in seasons 8 and 9, I believe. Of this pair, Reyes was the believer and Doggett the skeptic. Anyway, Mulder was eventually found. And soon after had to go into hiding, for almost the entire final season.
Over the years, we learned a hell of a lot of secrets of the conspiracy, but I don't think we could ever know everything, really. (Perhaps the greatest mystery was the question of where the hell Mulder and Scully got their awesome flashlights. I'd still like to learn the answer to that one.) But there was so damn much we did learn that it's practically impossible to keep it all straight or remember it all. And as is the common consensus among fans, the quality of the show declined in the last few seasons, especially without Mulder around. And even the best of the series... well, I find myself a bit less interested, in retrospect. The 90s are over. Still, it was pretty good. Very interesting, creepy, noirish, with more humor than you might expect (but no less dry).
I also need to mention that there have been two theatrical movies so far, Fight the Future (in 1998, while the show was still ongoing), and I Want to Believe (in 2008, six years after the show ended). Also, there was a short-lived spin-off series called The Lone Gunmen (in 2001), about a trio of conspiracy theorists who had occasionally worked with Mulder.
I also want to mention that I have a sort-of soundtrack called Songs in the Key of X, which has the theme music (which is nicely eerie and a perfect fit for the series), as well as a bunch of cool but dark songs by different artists, none of which I'm aware of having actually been used in the show. But it's one of my favorite CDs. Speaking of music, a song which isn't on the CD, and definitely has no real connection to the series, is Mulder and Scully, by the band Catatonia. And there's a song called David Duchovny, by Bree Sharp. I'd definitely recommend checking out both of those songs, if you're unfamiliar with them. And the show has inspired so many other things, it's just... a cultural icon.
Update: In 2016, the show returned as an event series, referred to as "season 10." It was followed by an eleventh season in 2018.