I'm one of the few that thought the original Blair Witch Project was scary.
You're not alone! BWP is underrated. Most of the complaints come from people who wanted a traditional horror "payoff". I thought it was pretty great.
Not doing a full top 10, but thought I'd add some that haven't been mentioned yet.
Texas Chainsaw Massacre (the first): precursor to the slasher films of the late 70s and 80s. It shows very little on screen violence but if you have a vivid imagination it's got some of the most disturbing images you'll ever see in your head. 80% chance of turning vegetarian after you watch this.
Carrie: while still overrated by some critics, IMO, the one big scene is enough to make this one a favorite. Most of the rest is kinda meh, but that one scene... Holy crap!
Halloween (the first): like Carrie I don't think this one deserves all the praise it gets, but it's still very influential. The first half of the movie is perfect for setting the scene, and it doesn't rely on jump scares. In fact the killer is kind of in plain sight the whole time, which is a different kind of scary. I'm not a fan of the second half because the killer just comes across as kinda dopey and incompetent.
Poltergeist: still the gold standard for supernatural horror. It scared me shitless as a pre-teen and growing up hasn't dulled that very much.
The Machinist: not true horror, more of a psychological thriller. But like Texas Chainsaw Massacre, if you let your imagination run wild it will show you some of the scariest things imaginable. It blurs the line between reality and nightmares.