Oh man, I'm a huge horror buff and can't choose just five. So I'm gonna cheat.

Movies that properly scared / disturbed me the first time seeing them:

Dawn of the Dead (2004)

The Babadook

The Fly (1986)

The Exorcist

The Ring

(runner up: 28 Days later and the first 15 minutes of 28 Weeks Later)

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Entertaining horror movies:

Evil Dead 2

Dog Soldiers

Dead Alive / Brain Dead

An American Werewolf in London

The Cabin in The Woods

(runners up: Attack The Block, Return of the Living Dead)

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Creature Features:

Tremors

Aliens

The Thing (1981)

The Blob (1988)

A Quiet Place

(Runner up: Feast)

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Horror / Thrillers:

10 Cloverfield Lane

Hush

The Invisible Man (2020)

Sunshine

Se7en

(Runner up: Don't Breathe)

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Lovecraftian Horror:

The Color Out of Space (2019)

The Banshee Chapter

Re-Animator

The Void

In the Mouth of Madness

(runner up: From Beyond)