Although horror movies can scare audiences silly for 89 minutes of a 90-minute runtime, it doesn't mean much if there isn't a final shot powerful enough to leave a lasting effect.
Unfortunately, because directors understand just how important an ending scare is to the genre, it's resulted in filmmakers and studios often falling into cliches when constructing a film's final image. Hell, over the past 10 years it's pretty much become obligatory to have one last jump-scare tagged onto the biggest movies (see the likes of Sinister or Unfriended as the worst offenders for this).
While this approach certainly leaves the audience on a shocking note, after years of overuse it's become a bit stale, and often feels like a tacked-on afterthought, unconnected to the movie that preceded it.
Instead, the scariest final shots in horror movie history are often the more understated ones. The ones that get under your skin and stick with you for days. The ones that act as a perfect culmination of a movie's themes. The ones that don't need to resort to a loud noise to scare the absolute s**t out of you.