Les Yay: Sun and Lunar, Sun and Akeno, Mawari and Sun & Mawari and Akeno all have moments. And the Class President tends to unwittingly have them with the other girls too, much to her dismay.
Moe: In one episode, Lunar's father, Gozaburo, and Masa (on orders) crossdress as schoolgirls and catgirls and act out moe stereotypes in an attempt to understand a girl's heart. It really is that horrible. Ends with Sun suddenly receiving cat ears, which everyone agrees is incredibly moe (before being blasted away).
The second half of the 2nd OVA, which does a Genre Shift from comedy/action to straight horror, borrowing elements from The Ring. Saturn will probably haunt your nightmares now.
That part of the TV series were Sun's family is knocked out by the creeper mermaid noble just because he wanted Sun for his own petty desires is startling for how serious it is. The terror on the Seto clan's faces as the special incense works to knocks them out and Ren cries out for her daughter to run away is sincerely frightening.