Richard Wagner- Much of the iconography and ideology of metal was initially popularized through the Ring Cycle. His reinvention of music as performance art was also crucial, and tends to be overlooked.
King Crimson- Fripp's early experiments in texture, tone and structure presaged what was to come in metal, keyed a move (of which metal was a part) away from blues mannerisms and blues compositional techniques.
The Doors- The first band to delve deeply into the darker side of consciouness (though often pulling back in fear at the edge of the abyss), and also, along with King Crimson, among the first to explore narrative and thematic structures rather purely cyclic riff centered contructs. "The End" is still heavier in the only real sense than 95% of metal songs.
King Crimson- Fripp's early experiments in texture, tone and structure presaged what was to come in metal, keyed a move (of which metal was a part) away from blues mannerisms and blues compositional techniques.
The Doors- The first band to delve deeply into the darker side of consciouness (though often pulling back in fear at the edge of the abyss), and also, along with King Crimson, among the first to explore narrative and thematic structures rather purely cyclic riff centered contructs. "The End" is still heavier in the only real sense than 95% of metal songs.