If we're going by pure jump scares, I'd say Halloween 4 has it, with Poltergeist a close second. But Poltergeist has better story and scenic design, so I give it to that overall. UM is third, but not by too much.
I think experience level and fear of the unknown determines a lot of what's "scary" to a person (and obviously, their personal tolerance to horror). The first time anyone goes through their first haunt, it's pretty overwhelming and terrifying. I still remember my first KSF maze back in my high school days (well before HHN even existed), and I have very strong memories of the fear I felt... even though looking back, it was probably one of their more mediocre mazes at best (Bait's Motel, if anyone else has a memory that goes that far back). As time goes on, and one gets more used to haunts and sees how they typically work, the scares are generally not as effective because you learn their tricks and limitations.
In a way, that's why visiting a new haunt is always exciting... you really are not quite sure where the scares are going to come from. One of the reasons I'm particularly excited about my upcoming first visit to the WB haunt... haven't never gone before, I'm not quite sure what I'll be in for. By the same token, after umpteen years and visit to HHN, I know exactly what I'm in for. And it's one of the reasons I give a slight edge to KSF... they're not afraid to try new ways to provide scares and creeps (the scene in The Depths where actors come from below comes to mind). They may not always work, but at least it's different, and keeps you a bit more on your toes.