I forgot about the scale too. While Slappy is the king, those weren't my favorite books. I'll need to think if 'best' and 'scariest' are one and the same. Figure that the consensus pick for best would be The Haunted Mask though.
The Texas Chainsaw Massacre (1974)
Certainly a lot of screaming going on here, and for the most part this thing, making pretty good use of a sub-90 minute run time, works. Have to figure that at the time of its release it played better than looking at it with a pair of eyes that sees how its style of horror has transformed and been cranked up to almost absurd levels in a modern context. Not particularly scary, but these slasher things typically don't get me that much, and I always appreciate that there aren't a ton of manufactured jump scares. As it goes on too you get the bigger performances, which I enjoyed. I did really enjoy this, but not too much to say about it.
On the Goosebumps scale, I would give this a Piano Lessons Can Be Murder.