I'll start off by saying I go to Halloween Horror Nights every year but this year have multiple groups interested. I plan on getting the express pass for the first time which is supposed to make the experience way better, but will also probably allow me to see everything in one night. So, I'm thinking of steering some of these friends groups to one of the other places this year.
Here's my own thoughts on these 3:
HHN always has great production quality and costumes, but it's starting to suffer from its success due to being crazy crowded. They're also starting to cheap out I think: reusing mazes and having less scare zones and there are maze special effects I remember from years past that I haven't seen recent mazes match up to, or maybe that's just nostalgia talking and the result of me going so many years in a row lol.
I recall Knotts being pretty good when I last went, but that was almost a decade ago. Iirc it didn't have as scary scare zones but I think I was impressed by their mazes. But I haven't been in so many years, I saw on the other site some people are now saying it's better than HHN due to crowd and cost issues, plus it being just as scary, making it better bang for your buck.
Fright Fest I went even longer ago. It was fun and even cheaper, but nothing really stands out in my memory except a cool 3D maze. It felt better for teenagers and younger crowds. But looking this year for the first time in forever, it looks they're starting to get some big names: Saw, Conjuring, cult Halloween movie Trick r' Treat, Stranger Things, etc. Good for them!