Mulder and Scully having an impromptu slow-dance at the very end of the "The Post-Modern Prometheus". That the episode itself was something of a CMOA for the season or series which earned it seven Emmy nominations and a parade of rave reviews enhances the good feeling. David Duchovny would go on to say that whatever came after, for him, that one moment would always remain his favorite.
"The Unnatural" is just one long CMOH.
Scully's passionate speech to Mulder in the first season episode "E.B.E." is so unexpected coming from her but also completely natural after all they've been through together.
"I have never met anyone so passionate and dedicated to a belief as you. It's so intense that sometimes it's blinding. But there are others who are watching you, who know what I know, and... *beat* Whereas I can respect and admire your passion, they will use it against you. Mulder, the truth is out there. But so are lies."
And the best part of it? The next scene reveals that Mulder has agreed with her in his own tsundere way. It also marks the point where Scully becomes the only person Mulder trusts (and vice versa).
"Folie a Deux", and Mulder telling Scully she's his "one in five billion".