Personally I don't enjoy books that explore the ultra dark side of humanity. King does the same in some novels, but many of his stories mix in the supernatural element and those books I'm good with and love. That's why I don't read authors like Jonathan Kellerman and James Patterson. The last book by King that I had to put down and didn't finish was Rose Madder; it really focuses too much on the horrors of domestic violence and the psychopath dishing it out. Too dark for me...
If you want to try some of his fantasy/horror stuff with focus on fantasy and supernatural I could suggest a few. I'm still exploring his works so keep that in mind.
IT - VERY dark book. Imo, he does a masterful job creating a town that has a sickness of the soul. If you've seen the movies you get the plot, the book isn't too different plot wise but one thing the movies didn't capture is the darkness of the town and why the antagonist was so attracted to it. Big book, it's a journey so be prepared.
The Stand - Has a underlying religious factor, most of his books do, some more subtly than others. But this is pretty much an end of days, good vs evil book. As with IT, the story brings in a diverse cast of protagonists that join together to fight the ultimate fight. Another big book, but like IT I never felt it was too long.
The Dark Tower collection - This includes books 1-7 and an interlude called The Wind Through the Keyhole. This is imo King's best work. There's fantasy, science fiction, a western, horror all rolled into one. Many character's from his other works appear in this one. It was my introduction into the King Multiverse (yes, after reading this you realize that's a thing) and got me hooked on his works.
The Shining and Doctor Sleep - One of his early works (1970s) and still holds up to this day. A story of a haunted hotel, a broken family and isolation. Vintage King horror here. Followed up just a couple years ago with the sequel Doctor Sleep, I'm not sure The Shining needed it but it worked and I loved it!
All these books are his early works with the exception of The Dark Tower which were published from the early 80s to the early 2000s. If you start with some of these you'll realize why people love him. If none of these resonate with you, maybe he isn't for you and that's cool too.