His short stories are generally his strongest work, in my experience. Though I liked IT quite a bit (aside from the scene everyone talks about, of course). The Stand had some good scenes, the tunnel in NY I remember in particular, very much felt like a zombie movie scene for a bit there, but ultimately I found the book lacking, mostly due to the ending of the Vegas plotline. I get that King was trying for a Tolkien-style eucatastrophe, but it was too deus ex machina for my tastes.
I'd suggest trying the first Gunslinger story, it's more a novella length iirc, or at least my version is. I think he revised it later. That's a pretty solid introduction to King, and well written. It's actually what got me into reading King, after I tried one of his books and didn't care for it. The first 3 Gunslinger books are solid for most people, I find.
And if you want more horror, try Skeleton Crew. I haven't read all the stories there, but The Jaunt and The Raft honestly fucked up my head for months. So much that I generally steer people away from them (the Jaunt in particular) unless they really want that sort of thing.