This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Blue Comet SPT Layzner
This show is basically Real Robot Genre meets Survival Horror. The first half is one Hope Spot after another as the SPTs tear through the Earthling technology like tissue paper. Among the first is the nuclear annihilation of the American Mars base and the ensuing shots of soldiers who tried and failed to get to the shelters in time.
Scenes of Gosterro gleefully gunning down helpless Earthlings can be pretty scary.
The AI-controlled Skullgunners give the Cosmic Culture Club one hell of a fight.
The way in which the Gradosians decide to subdue the Earth's society once and for all is pretty horrifying; use a Kill Sat to bombard their planet with UV rays.
The tyrannical rule of the Gradosians over Earth in the second part becomes even more nightmare-inducing when the audience remembers that it's almost surely based on what Imperial Japan did to Korea.