Here's a controversial but extremely influential one, 1982's Blade Runner, directed by Riddle Scott in a seemingly-ignored and damned multi-production effort (U.S., the U.K. and H.K.'s Shaw Brothers Studio), the movie itself hasn't been really forgotten due to its combination of expressive, very well made assets but still all used by a somewhat corporativist spirit driving what would be a fucking mess of a product, soundtrack included.
In my opinion the movie does almost everything right at a very high level but has a couple of really egregious mistakes that make it odd to palate, one is the director being a pushover ala Walter Hill (or a retarded conman) which resulted in many versions and interpretations... and second the frankly mediocre acting of the main character portrayed by Harrison Ford, picked after the U.S. investors imposed the jew-favorite actor in a tug of war; to have a forgettable main interpretation in a noir movie is usually a damning thing.
But anyways the greek electronica pioneer by the artistic name Vangelis was in charge of scoring the super-budgeted film, being contacted in his London studio by Scott after finding success in the score for the jewish fantasy film Chariots of Fire. While the movie "bombed" (as in it didn't make Star Wars money) it quickly became a "cult" classic due to its high standards in everything (but Ford) yet in my opinion most of this cult status came from the movie being actually hard to get ahold on after its initial release: Prints were being recalled out of butthurt, it didn't screen in non-massive markets or was shown only late-at-nite, TV screenings were rare, word in mouth exacerbated most of its already-cool qualities (to the point of overrating) and most importantly for our subject here, the emblematic soundtrack wasn't released until 12 years later.
Much akin to the great Koyaanisqatsi the soundtrack (and SFX too in this case) became as iconic as the moving pictures on the screen yet the big effort made by the athenian wasn't fulfilled in the open market for some reason, and actually to this day the complete OST has not been released... just like Koyaa. Vangelis retells how his scoring method consisted of seeing first-hand tapes from the sets with finished scenes or outtakes, Ridley liked to promote the movie in the middle of production which was odd back then hence why he had tons of tapes, but the greek inspired himself mostly from the "mood" made by the sets and the lighting plays commanded by the cinematographer Cronenweth, not to mention the physical recreations of Syd Mead's drawings.
The guy used most of his keyboards around and because the city portrayed was a multiculti hellhole he also had to take hand of different scales and sounds from the world, mostly East Asia and the Middle East. He recorded stuff for almost every scene but as mentioned most of this was never released, so what happened? Bootleg galore, this soundtrack has as much versions as the movie itself, fan-edits included and it's an understatement to say this film was influential for the synth-focused musicians who are already considered a bit more mental about their pursuits.
One of those efforts, perhaps the most famous one, is the Esper Edition which had a "final" one around 2007 (this post) when the Final Cut was released along with the second official version of the OST. Basically some skilled dudes used tons of different VHS and DVD versions from various releases and bootlegs, used the official soundtracks and some bootlegs sneaked out by somebody (either an engineer or te same greek) then it was mixed all together to create versions as close as possible to the movie YET they made an interesting decision, instead of leaving them as individual pieces without voice overs they decided to mix them together so it could be an "integral" journey, like a concept album/watching a movie unlike the big record releases.
This long fan release edition, dubbed the Retirement Edition, quickly became a sought-after item because they printed it in very few numbers either as a play on how awful it was to get products related to the movie or because they didn't have money/were assholes. The complete set fetches hundreds of dollars and someone around did FLAC rip the first two discs which are the Score itself, the others are pretty cool bonuses but haven't seen them in the open yet.