In one episode which parodied The Wicker Man, there is a secret society worshiping the bad guy who you can see the organs of, and two of its main members are a sweet couple who had been around for four seasons before this came up.
Skeleton King creates this in-show for the team in In The Grip of Evil.
Oi, several episodes were Nightmare Fuel, Circus of Ooze anyone?
Pretty much half the final season. Lesse... Secret Society for starters. Then Fire of Hate/Soul of Evil. I mean, between Sprx, the resurrection ritual, and Skeleton King's new look? Good grief.
Don't forget Incident On Ranger 7, Ghosts of Shuggazoom, and Demon Of The Deep.
The Probe from Ghost in the Machinder.
The Skeleton King Droid. Relentless, brutal, and nigh invulnerable. And then there's that voice… *shudder*
OH GOOD GRIEF SKELETON KING! WITH THE TORTURE CHAMBER AND ALL THE MONKEYS RELEASING HORRIBLE SCREAMS!
Virus!Gibson was incredibly creepy in "Thingy".
Here's a nice picture for you. A space ship is de-powered and lost in outer space. The team finds the ship and learns that the ship's crew had been there for possibly decades as gray husks and walking corpses that moan and shamble around a la Dead Space style. The team defeats them by breaking their glass face masks on their helmets which, because they are outside the ship, caused the putrid air that's been festering in their bodies for decades to be forcibly sucked out. The bodies are so fragile that the suction causes their faces to implode. You know, FOR KIDS!