My favorite show of all time for many reasons:
1. Amazing main characters with incredible chemistry
2. Unpredictable episodes - you never knew how an episode would end it may be a situation where whatever issue at hand gets resolves or solved, there may be no conclusion at all, there may be a "bad" ending or it may be a "to be continued" which sporadically would happen. As opposed to another show it gets compared to at times, Fringe, which had an extremely formulaic episode format.
3. Interesting main story that was SLOWLY trickled out over many seasons which made the surprises and twists earned. Each season there would be some main story episodes and some monster-of-the-week episodes so it was a drip feed of main story that I think worked better than shows these days like Westworld where they constantly have to be doing more shockers and surprises to continue to top themselves and they lose their value.
4. Lot of episodes per season - many seasons had 20+ episodes.
5. Great comedy at times. X-files doesn't get the credit it deserves for some fucking hilarious episodes and running bits on the show like Mulder's porn stash and other things.
6. The male/female duo were allowed to be friends who deeply cared about each other instead of a romance which you almost always find now. Yes it technically does go into romance late in the series but the meat of the series it isn't like that.
I fucking love that show so much.
I've got this and unsolved mysteries collection.