I love this topic from Long and Short Reviews. For many years my favourite film was "Heavenly Creatures", and I still do love it. More recently I discovered "Hell House LLC" and its sequels, and those now take precedence. However, the one point of criticism I have for "Hell House LLC" also applied to "Heavenly Creatures" - the director's cut is better. yet harder to find.
"Hell House LLC" uses the mockumentary/found footage format to great effect and manages to evoke real dread, even though from an analytical perspective it's probably quite simple. It does, however, tread a fine line with how much it reveals of what's really going on, and possibly falls slightly on the side of not revealing enough. The director's cut does a better job, as it reveals a little more without going over the top, and as a result is scarier than the original - however, the director's cut has only just become available on Shudder after waiting ages for it, and the DVD and blu-ray are US only.
The film also has one issue which annoyed me the first time I saw it - every haunted house film has to have a good reason why they're not leaving the house, particularly when it's found footage, but in "Hell House LLC" they literally turn the camera off for this conversation and then switch it back on to say "You're right, we can't leave." The reason is revealed in a later film, but this seriously irritated me at the time.
I'm interested to see what everyone else is going to come up with!