Some great shouts upthread,
Maybe these would be classed as niche hidden gems, rather than forgotten ones but 3 I love...
The Vanishing 1988 ( creepy french thriller, a horrible story told in such a mundane everyday banal way that just adds to the horror)
Timecrimes ( 2007 spanish time travel loop nonsense with added mindfucking trippy hilarity. Done on a miniscule budget that puts shame to every multi-million special effects gross-out of the genre )
Careful (1992 Guy Maddin film with a 20's/30's german expressionist style 'beware the judderman' low budget hilarious arthouse craziness. Full of invaluable lifetime advice such as 'keep a lid on it, don't be rash, never go too close to a baby with an open pin, don't put so much pepper on it, don't stand too close to a walnut tree' etc etc
)
The Vanishing endured the indignity of a vastly inferior shit american Hollywood rework. Timecrimes was rumoured to be a remake project for Tom Cruise (thankfully we we've been spared that one so far). I doubt anybody in their right mind would even dare thinking about attempting a remake of Careful.