Usually short-story collections, especially those by the same author, often receive a three from me. This is because they are almost always a combination of the good, the bad, and the indifferent, following the bell-shaped curve of the normal distribution. However, this collection is different. King's early stories in this collection are filled with the excellent, the very good, the good... and a few that are mildly good. The distribution leans heavily towards the terrific.
It has been a long time, but many of the stories still linger in my mind. In my opinion, the seminal one is The Boogeyman. This story points to the basic concerns behind King's writing and any horror story in general. The author does a great job of walking the tightrope between psychological horror and pure, gut-wrenching terror, maintaining the suspense until the very end.
Another story that still haunts me is The Children of the Corn. The wild children of the cornfield and their twisted religion are one of the finest examples of creeping horror in the traditional sense.
I am the Doorway and Sometimes They Come Back are two other stories that really scared me. The remaining ones, although not as frightening, gave me pleasant shivers and "delicious nightmares" (to borrow a phrase from Alfred Hitchcock). I return to this collection again and again whenever I feel that life has become too safe and dull... just to remind myself that the boogeyman is always close by, behind the closet door.
(P.S. BTW, if you ask me to pick one story from this collection as my favorite, I would choose the only one that is \\n not\\n a horror story - The Last Rung on the Ladder. The reason is personal. I too have a kid sister like the protagonist of that story, who knows that the hay will always be there.)