My absolute favorites are mostly your standard entries that you’ll find on plenty of top 10 lists:

• Halloween
• Alien
• Scream
• Night of The Living Dead
• An American Werewolf in London

Apart from these, I’d recommend

• The Wailing (2016, 156 minutes): a Korean movie about a detective trying to find the cause of a mysterious disease running through a town
• Barbarian (2022, 103 minutes): this movie falls in the very weird “AirB&B” sub genre of horror. It has some scenes of incredible tension, one of the best (and well-earned) jump scares, and some great humor.
• The Return of the Living Dead (1985, 91 minutes): a horror-comedy zombie movie with some incredible practical effects

For my least favorites, I’d probably put Slenderman, Meghan is Missing, Winchester, and Ghosts of War.

I think Sinister is one of the scariest horror movies I’ve ever seen