This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Independence Day
The destruction of NYC, Washington, and Los Angeles, which ends the first act. Imagine being on the ground, watching those hovering spaceships open up at the center and shoot a beam down, believing it to be a welcome message from the aliens. Then the building the ship is under disintegrates inhumanly fast and a wall of fire starts moving outwards in every direction, obliterating everything in its path and hurling cars away. It is a fireball you can't outrun. Critics at the time praised this scene for its haunting realism, which became even more hauntingly real on 9/11 five years later. Marketing for Independence Day: Resurgence reveals this happened world wide in 30 other cities; imagine being in London, Moscow or Paris, watching that wall of fire advance, destroying everything in its path.
The novel effectively describes the devastation as "a hurricane, a flood, and an atomic bomb all rolled into one."
Not to mention the War of 1996 website shows the course of the alien invasion. Every twelve hours, the ships move to their next targets and destroy them. Then they do it again. Like clockwork.
General Grey: If you calculate the time it takes them to destroy a city and move on, we're looking at the worldwide destruction of every major city...in the next 36 hours.
Following that, the alien communicating via Dr. Okun and making the aliens' motives horribly clear:
President Whitmore: Can there be a peace between us?
Alien: Peace? No peeeeace.
President Whitmore: What is it you want us to do?
Alien: Die... dieeeeee...
Some meta-context is needed to explain why this was so frightening. While films with aliens killing humans were quite common even before this film, most of these were slavering B-movie monsters, or otherwise incapable of communicating with us. Aliens that could talk to us were usually friendly. The aliens in this movie are among the first notable aliens shown to be able to communicate with us meaningfully for the sole purpose of telling us they're gonna kill us.
Then the alien uses a psychic link on Whitmore to show him about its species. The novelization goes into more detail, but it's Fridge Horror at best on what he saw: countless battles across countless worlds, many species slaughtered, whole civilizations scorched, the invaders land and establish their own settlements, then they consume the planet's resources until there's nothing left and go back to the mothership for their next journey.
The point inside the mothership where Hiller and Levinson fly over a staging ground with thousands, if not millions, of alien soldiers assembled, waiting to board transport ships. Hiller speculates that they're likely a ground invasion to wipe out any last vestiges of resistance on Earth, and given the numbers of the invaders, it really drives home the point of what will happen if they fail their mission.
The novelization goes into a lot more detail about nightmarish elements:
Russell has a flashback to his abduction. One night, he's out working on an airplane engine in a hanger. Then suddenly his body is paralyzed. He obviously thinks it's a stroke, but then he begins hearing the aliens say "You will not be harmed." Then they break out the probes, all while Russell is looking out a viewport at Earth from the lower atmosphere...
We also are treated to a flashback of the Roswell crash. Even though the aliens are utter assholes, their crash is still terrifying to read. Their carrier ship jumps away, as they were on the verge of being discovered, and because of that, their power systems quickly died. One of the pilots was killed in the crash, and another tried to crawl away from the site, but was attacked by coyotes.
The post-war world. The invaders have been resoundingly defeated, but the damage has been done. Imagine 1945 Europe on a global scale. 108 cities, from New York to Shanghai, Berlin to Moscow, Riyadh to Pyongyang have been obliterated. Nations and their militaries are in ruins. Millions, likely billions, are dead and millions more are now displaced. The cost of rebuilding and the sheer scale of restoring what has been lost is incalculable. Factor in how long it took to rebuild Ground Zero after 9/11, then scale that up to all of New York and multiply it by 108.
The alien city ships are pretty terrifying. There's no beauty to them; they're just giant work barges built to do one job. They just hang over cities ominously with no broadcast of their intentions. Then they open, bathing the city below in greenish light, fire their weapon and utterly annihilate the city, close, and move on their next target.
The giant fireballs in the sky that form from the ships entering Earth's atmosphere. "Harbingers of doom" describes them perfectly.