This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Power Rangers Dino Thunder
Mesogog, plain and simple. Possibly the most evil being since Lord Zedd.
He's actually worse, because Lord Zedd still had some sense of humor to him that made him seem more Faux Affably Evil. Not with this guy, AT ALL! He's one of the few villains that didn't have an ounce of comic relief in him, or a softer side like Ransik's love for his daughter. (He lampshades this in "Back In Black" after a captured Tommy fires off another badass snark: "There's that wit again. If I had a sense of humor, I might actually appreciate it.") Never once during the entirety of Power Rangers Dino Thunder does he show the slightest traces of compassion and humanity (barring a single occasion where his human half intervenes on behalf of his adopted son- whom Mesogog himself is about to kill), and when his subordinates fail him, he doesn't throw a tantrum, he doesn't scream about how much of a headache he has - oh no, he tortures them via Mind Rape.
Adding to his general creepiness is how well the facial prosthetics used for him work. Mesogog's suit looks extremely realistic, complete with a working mouth with internal details, makes him all the more terrifying.
That monotone of his was scary as well: He never raised voice, not once. Creepy Monotone all the time.
The only time he raises his voice is when the Rangers discover his true identity.
He did get significantly crazier after he separated himself from Anton Mercer, however.
What's scarier than Mesogog? Some of Anton'stransformations into Mesogog. No sparkly effects, just painful-looking Body Horror.