Labyrinth! Fantastic movie that introduced me to David Bowie (may he rest in peace). It was before my time, so it was a bit surprising that it didn't do so well when it was in theaters.
I grew up watching this movie in the 80's. It wasn't until later that I found out it was a flop. When it came out I was a bit too young, but I did see it at the cinema last year at a special screening after Bowie died And yeah, my Bowie obsession started with this film.
Phantom of the Paradise (DePalma, 1974) Quatermass and the Pit (1967) Colossus: The Forbin Project (1970) Zardoz (John Boorman, 1974) The Loved One (Tony Richardson, 1965)
I don't know I'd classify all these as either overlooked or cult/B...though I suppose they are. They're all movies I've watched literally dozens of times each, loving them more every time. Interestingly, they're all from my late adolescence/early majority period. That must have something to do with it.