This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Plants vs. Zombies
Okay, sure, eat my brains. As long as it means I don't get nightmares from this.
Even though he's on your side, the Tangle Kelp is a mass of kelp that inexplicably has red eyes. But the scariest thing about him is what he does: he pulls a zombie underwater and they never show up again.
Jack-in-the-box Zombies, especially if you are facing them in the fog levels where you cannot see where they are coming from. They explode too.
"A huge wave of zombies is approaching!", accompanied by a Scare Chord too!
If a zombie manages to break your defenses, you get to watch it stroll into your house, followed by the hideous sound of it crunching into your skull along with a gut wrenching "NOOOOOO!"
In Plants vs. Zombies 2: It's About Time, the game over screen also features a brain on a plate with a hideous bite taken out of it.
The Gargantuar. It's big, it can survive an instant kill, & it smashes your plants with 3 different weapons* A telephone pole, a wildlife crossing sign, or another zombie instead of eating them. Let's also not forget how it comes up from the ground in the Garden Warfare gameplay trailer...
If you thought the Gargantuar was bad, we have the Gargantuar Prime from the Far Future. It's half as fast as a Gargantuar, but it doesn't even need that speed as it has Eye Beams which can incinerate a random plant in your back lines! Nothing is safe from these guys!
The hidden minigame Ice Level, which is an extreme case of Nothing Is Scarier. All there is is an empty field. No sun drops, and you are automatically given a set of 6 plants. None of these are Sunflower, so you can only place a Wall-nut. Zombies never appear, and after a while, the Game Over music plays, but you don't actually die. The level was actually going to be a level in the game, and actually playable with different features, so this is mostly leftover data. Doesen't make it any less disturbing, though.
Most of the time, the zombies look far too goofy to be all that creepy, but in Stage 4-10, you have to fight off their waves in complete darkness. The only time you can see is when lightning flashes. The music is absent and the only sound there is, is the rain and thunder, and the zombies calling for brains. Under these circumstances, they actually are unsettling and it's even moreso when you hear them chewing on one of your plants, making you wonder frantically which row needs more defenses.
In Plants vs. Zombies 2: It's About Time, Big Wave Beach Day 32, one of Zombot Sharktronic Sub's attacks is creating a turbine engine that sucks in plants and zombies alike. Nothing graphic is shown, of course, knowing the nature of the game, but it can still create a vison of a rather horrific scenario in one's mind.
In the battle against the Zombot Aerostatic Gondola, Zomboss occasionally moves to the fire tiles, grabs a bag of sand, stares unnervingly at the plants, who are hopeless due to the fact that you can't stop the attack, then lets it go, torching up the plants.
The Punk Jam can be a massive Oh Crap! moment to some. It's not the music that's scary, it's the fact that every zombie on the lawn now moves and eats three times as fast during the Jam. Combined with the Punk Zombie who can kick your front lines away during the Jam, and it becomes a situation where the zombies easily overrun you if you're not prepared.
After defeating the Zombot Dinotronic Mechasaur, Zomboss mentions he has no other choice than to launch "Project Paradox". This could be really bad.
The Newspaper Zombie in the sequel, after his newspaper has been destroyed. He moves AND eats so fast that he literally destroys non-defensive plants in less than a quarter of a second, and can easily run through your defences if you're not prepared.
Before battling Zomboss in Modern Day Day 32, he mentions that Crazy Dave and Penny have broken the time-space continuum. Penny starts to worry about their actions, and to top it off, Dave starts talking normally...