This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Smile Pretty Cure!
Episode 32; seeing all of the Cures smiling with Dull Eyes of Unhappiness is pretty unsettling. The dissonant music doesn't help.
That was actually a scene lifted from Yes! Pretty Cure 5; fitting, as Joker is an Expy of Kawarino, the one who brought that scene with the similar 'Nightmare Mask'. However, Joker gives a more nightmarish continuation and conclusion: while Nozomi brought her friends back to their senses with mere words, Miyuki is forced to fight an Akanbe in a losing battle alone while her friends just watch with the same expression, and every attempt of her calling out for them to wake up just fell to deaf ears. That's like you're in a nightmare and none of your screams of help do good to get you out of it, even if helpers and friends are nearby. Good thing Miyuki is a Plucky Girl and her refusal to stand down eventually broke the spell.
Many consider Joker to be this himself. Especially some of his expressions.
In episode 40, he summons Wolfrun and starts to berate him for failures, his back to him the whole while. Wolfrun gets angry and defensive...until Joker finally turns around to reveal a mouth full of fangs and glowing red eyes far wider than he's ever made them before. And then he makes this expression twice more in the next two episodes. Meep.
Episode 43; while Joker Breaks Reika By Talking and mentions her two selves—Cure Beauty vs Reika, he shows her a giant mirror. Beauty looks into it, seeing her reflection...and Joker briefly takes control of it, so now her reflection has his creepy Slasher Smile and wide eyes. And talks in his voice.