Good list.
I agree that The tent scene in blair witch was The creepiest.
Like many People, i do consider exorcist iii superior to The first and The hallway scene gets me every time as well.
To me it is easy to list the scariest Scenes ever because only a selected few films have managed to really scare me. So, this is not a favourite list, it is a scariest list. There are films that i Love that couldnt scare me less. Some of these films have more than one scary scene.
So let's go, not in any particular order:
Jaws: Ben gardner's Head, you Will need a bigger boat, The pier that chases a Guy swimming
Alien: Facehugger, dallas, brett, you are my lucky star.
Changeling: Wheelchair scene.
Poltergeist: Mirror scene, swimming pool scene, tree scene, clown scene. These scenes haven't aged well, so I don't find them scary anymore.
Sixth sense: Angry suicidal mum.
The resurrected: Mutant corpse found in river.
Blair witch: The tent scene, voices of children outside, the trailer.
The fourth kind: Guy under hypnosis yells unexpectedly, milla yovovich jumps floating off her bed.
P.a.: Door opens in bedroom, movement downstairs, lights turn on, Kate stands up in bedroom stares at bed and sheet floats away.
The ring: Samara comes out of the well, and out of the tv set.
The others: Nicole Kidman walks into an empty room with old furniture, she finds the photograph of dead people, the servants try to enter the mansion.
"It doesnt matter what Bram Stoker has told you dead people don't come back from their graves"

Angel Heart was creepy when Bobby is revealed to be the devil.
RIP
Jim Morrison
1943-1971

To me The Blair Witch Project and The Exorcist are not scary at all. I never could understand how anyone could see anything scary in either of them.
Amityville Horror had the yellow eyes in the window and I thought that was more scary than the part you mentioned. And the stranger that came to the door was creepy too.
Communion had a few scary scenes - the alien peeking around the corner mainly.
I thought the hallway scene in Exorcist 3 was scarier than anything in the original. In fact I thought it was a much better movie.
Bad Ronald had a scene much like the one in Black Christmas with the killer peeking through a hole in a wall.
The Entity and The Changeling were both suitably creepy.
I haven't seen Paperhouse.

I like your list. My top ten:
(1) THE OMEN (1976): Graveyard scene with the dogs.
(2) THE SILENCE OF THE LAMBS (1991): In the basement of Buffalo Bill.
(3) ALIEN (1979): Chest burster.
(4) JAWS (1975): Ben Gardiner's boat.
(5) THE CONJURING (2013): Basement clap-clap.
(6) SCREAM (1996): Casey's demise - the entire opening sequence scared the crap out of me.
(7) THE RING (2002): Rachel falls into the well.
(8) HALLOWEEN (1978): Laurie discovers the fate of Annie, Linda, and Bob - and comes face to face with the Shape.
(9) ALIENS (1986): Ripley and Newt vs. the facehuggers.
(10) CLOVERFIELD (2008): In the subway tunnel.

The Sixth Sense - Angry suicidal "mother" in the kitchen.
Great one. That scene traumatized me as a kid.
CG gore is the worst thing that has ever happened to the horror genre.

Good list. Yeah, this topic comes up every so often, but it's been awhile - and it's an always-changing topic for me. I call these my "creepiest scenes", and here's my top 15 right now, in no real order (too hard to cut/narrow down to 10):
The Shining - Twins in the hallway.
The Blair Witch Project - Final scene.
[Rec] - Final scene.
The Exorcism of Emily Rose - Emily on her dorm room floor staring up at her boyfriend.
Noroi: The Curse - Final scene with
the little boy/Kagutaba.
The Innocents - Miss Jessel in the reeds.
A Nightmare on Elm Street (1984) - Nancy's school dream scene.
Lost Highway - Mystery Man at the party.
The Orphanage - "Knock on the wall" with the
ghost children.
Kairo - The wobbly ghost scene.
Mulholland Drive - Diner scene.
Insidious - "Tiptoe through the Tulips" dancing "boy" scene.
The Sixth Sense - Angry suicidal "mother" in the kitchen.
A Tale of Two Sisters - The entire girl under the sink scene.
Twin Peaks: Fire Walk With Me - Bob looking for Laura's diary in her room.

Good list!

Nice list. #5 gives me chills thinking about it, though the scene where Clare is packing and the final scene are horrifying.
Our songs will all be silenced, but what of it? Go on singing.
Orson Welles