>>11888551 (OP)
The grand daddy of the RPG genre the same way Black Sabbath's S/T was for heavy metal music. Cover art also goes pretty fucking hard for both with a distinctly 70's occultish vibe.
Both are also greatly overshadowed in popular culture by their follow-ups, Paranoid and Ultima I respectively.
>basic but elevated by great fundamentals >kickstarted a genre >spawned a million copycats >people still doing the same thing decades later (.wads - Stoner Rock & Doom Metal)
The answer is DOOM.