I dropped this series at season 8 back in 2012 because I can't stand John Dogget, but aside that, this series is awesome. Alien, vampire, ghost, zombie, weird mutant, psychic, angel, god, demon, time travel, doppleganger, the intro theme, how cool Fox Mulder is, how I developed a crush with Gillian Anderson. As much as I love Twin Peaks, I miss being in love with The X-Files.
I looooooooove The X-Files. There a very special attribute that I really love about it, each episode is different from the other, you can watch a whole season and never feel like it's repetitive. The thing is sometimes that makes the seasons very inconsistent in quality because you get a (very) mixed bag of good and bad episodes.
It's biggest downfall was that it never had an end-game or a proper plan for it's conspiracy/alien/UFO/virus/ arc and it rewrote it's own rules numerous times and killed characters never stayed dead. The lead actors were a phenomenon in themselves though and the supporting cast of shady government officials were better than the show deserved (especially Mitch Pileggi). Funniest to watch when offbeat and strange.
On a clear day, I rank the X Files above Twin Peaks as my favorite series of all time (on any other day,, they're even): in a way, TP is "only" the exceptionally successful TV adaptation and development of a pre-existing universe (Lynchtown/Blue Velvet). The X-Files started almost (see above) from scratch and set a new standard in TV drama, paving the way for the Sopranos ("only" my 3rd favorite series of all time).
The greatest television show ever made in my humble opinion. Blending science-fiction with the paranormal, the lead character's emotional journey through their quest to find the truth produces one of the most egalitarian, intellectual and subversive partnerships to ever come out of film.