The climax to Robotnikland has this in the form of a haunted roller coaster, the afore mentioned Hellevator, a grotesque landscape with demons laughing at the villains, a door that sends them into a torture hall, Scratch's shadow getting decapitated (and putting his head back on!), a hallway of axes (where Scratch pulls out his heart), and fire breathing Badniks. The facade of this nightmarish landscape? Sonic's head!
"Spaceman Sonic" features a rather freaky slime alien thing. While it has some Nightmare Retardant moments due to its cold, it nevertheless is a Cyclopean monstrosity that seeks to eat Sonic and Tails after having nothing for thirteen years.
Scratch and Grounder melting in Birth of a Salesman while still conscious!
In one episode, Sonic has to decide if he wants to free Tails or people who got captured. He has to deliberately lose a race to get Tails back. While he was sadly trudging along, you hear Robotnik repeatedly yelling "YOU'LL NEVER SEE HIM AGAIN!" And it's echoed.