I am currently running through the Mythology episodes of the show so I am caught up for S10 when it airs here in the UK. A lot of well written, very dense TV episodes, and thus far making pretty good sense given it was made up on the fly. However, my memories of S7-9 are highly fuzzy as I have, thus far, never revisited them since original airing, so I will focus my list to S1-6 alone.
Not sure I can actually put them in order either, given my mood greatly affects any episodes standings, depending on whether I want horror/SF/weirdness or just laughs. So in episode order then.
Still the episodes below are all 5/5's for me:
Tooms - S01E03 - Nothing creepier than a man who can fit down a chimney to kill a man. Don't show the kiddies this one or they'll start looking at Santa in a whole different way.
Ice - S01E08 - one of the best "bottle" episodes out there. Plus Scully loses her shit. Love it
Beyond the Sea - S01E13 - Dourif and Anderson kill this, heartbreaking and terrifying at the same time
Duane Barry/Ascension/One Breath - S02E05/06/08 - When the show took every element of its storytelling power and slamdunked it home
Irresistible - S02E13 - At times feels like a trial run for MILLENNIUM (as did Howard Gordon's "Grotesque" in S3), yet this is a truly terrifying piece of TV and Carter ratchets up the tension inch by inch til it's almost unbearable. Chinlund's "Donnie" is probably only surpassed by Hutchinson's "Tooms" as creepiest Mofo in the X-Files verse.
Humbug - S02E20 - The first time the show really tried quirky. And thanks to Darin Morgan, set a precedent that the show could be deliberately funny, whilst still holding true to itself.
Anaszi - S02E25 - The first time I remember shouting at the TV blaming it for all my ills for three months or so.
Clyde Bruckman's Final Repose - S03E04 - Darin Morgan, plus Peter Boyle, plus banana cream pie. Perfection
Oubliette - S03E08 - truly stunning performance by Tracy Ellis and a wonderful examination of abduction, played through Mulder and his loss.
War of The Coprophages - S03E12 - "Her name is Bambi?!" alone gets this a 5.
Pusher - S03E17 - Vince Gilligan's first enormous episode. Probably the single most tense episode of the show's entire run, and Robert Wisden taunts and teases so very, very well
Jose Chung's "From Outer Space" - S03E20 - I remember the day after this aired in the UK so well. The show was still in water-cooler territory and, whilst on an internship at a Chemistry Lab, filled with crazy, intelligent chemists, STILL a whole bunch of them came running in the next day, screaming "What in the FUCK was that?". Took a few viewings for it to all sink in and it is probably the most insane TV has ever got and still made perfect sense structurally, character wise and also, sliding in nicely along the Mythology.
Quagmire - S03E22 - I just love the silliness of this episode. Plus, Englishman learnt a song about a Bullfrog called Jeremiah
Home - S04E02 - Morgan and Wong were well known to push the boundaries on the show and, regularly, would throw EVERYTHING at the wall just to see what stuck. This is a great example of them splattering the wall and coming out with a Pollock.
Paper Hearts - S04E10 - Huge airs of Robert Harris in the villain, but plucking and snapping those heart strings of Mulder makes for sensational TV
Memento Mori - S04E14 - Just finished rewatching this today. Carters purple prose sticks out too much in Scully's diary voice-overs, but everything else is magic, from Byers "sexy black" espionage outfit still including a tie and overcoat, to the idea of the Clones bucking the plan to try and save their mothers. Emotion without maudlin manipulation.
Small Potatoes - S04E20 - At the time I felt like this was Gillian mimicking Darin Morgan, though brilliantly. After watching him continue to pull out comedic gem after gem following this, it was clear that this was ALWAYS Gilligan's style. Like Post Modern Prometheus, it does have a rather bitter after-taste of "it's not rape, it's funny" about the actions of the "villain", but at least in this one he is caught and punished.
Bad Blood - S05E12 - Gilligan owning all of TV for 44 minutes
Drive - S06E02 - Ha, Gilligan again, but back to his thriller roots. Immensely tense, wonderful back and forth between Mulder and veiled racist Crump, perfect denouement and very cleverly shot.
Triangle - S06E03 - Carter regularly gets a LOT of flack for his writing, yet the vitriol (if you must) should be spared for the Mytharc and its pompousness. Carter, when he wanted to, knew how to have fun with the typewriter and the camera, and in S6 he fooled around twice. Once here, playing with time, character, structure and alot, ALOT, of fun camera techiniques, and again with "How the Ghosts Stole Christmas"
The Rain King - S06E08 - Not on many people's radar, but Jeff Bell's first foray into the X-Files was just so sweet, so good natured, that I couldn't help loving it. For a show that is regularly so dour, so focussed in its earnestness of "selling the idea", I loved seeing the show not just play a comedy, but a whimsical one at that. Plus, hailstones in the shape of hearts. Plucked my romantic strings.
Two Fathers/One Son - S06E11/12 - the end of the mythology for many people, and certainly the end of a real cohesiveness and pulsing direction for it. Yet, still, one must have realised that the end of teh Syndicate would never be the end of the arc. Wonderful to see so many threads come together and then be torn asunder. Couple that with the deaths of so many well hated faces and there is something distinctly cathartic about this two parter
The Unnatural - S06E19 - Duchovny brings back the whimsy. Aliens love baseball. Why not. Touching, heartfelt and exceptionally well directed. Boy had game
Hugo - Yeah, sorry, could only get it down to 23. There are many many others that I want on that list too