I know, already tackling one of the big boys. The Shining is often mentioned when people are asked what their favorite horror movie is. I would argue that it is one of Kubrick's most straightforward movies since his early war and noir films. Stephen King got quite the boost of popularity due to it (even though it does not follow the plot of his equally compelling novel). This movie is permanently enshrined in the Horror Hall of Fame.
So, what makes it good? Only one person is killed by the antagonist (and that character is not in most of the movie) so it isn't a high or memorable body count. Shelly Duval screaming a lot is kind of irksome. I would even argue that Nicholson flubs the part due to his limited range. As many others have suggested, he comes across as insane before he ever even takes the job. You don't see a descent into madness so much as an escalation of it. Even the plot isn't all that novel, isolation makes a guy go wacky with an ax.
So why is this considered one of the best horror movies? Why does it scare people? I would argue it is 100% due to the creative choices of Kubrick. Starting with the soundtrack of atonal noise that puts your guts in a knot and carrying through to the barrage of soul-scarring imagery encountered throughout the movie, I believe The Shining scares through impressionism more than literalism.
The shot of the blood exiting the elevators, the twins who go from butchered to not and back, the friggin guy in a bear suit blowing a dude in a tuxedo! All these images worm their way into your head and disturb you in ways a simple ghost story can't. As a drama about alcoholism and abuse, it is quite powerful as well.
I argue that The Shining is frightening because of how it is presented rather than what is presented. If you want to watch two hours of assholes describing their theories about the movie, watch the documentary Room 237.