Final answer:
Berlioz's most influential work was Symphonie fantastique.
Step-by-step explanation:
Berlioz's most influential work was Symphonie fantastique, a groundbreaking symphony that showcases his innovative use of orchestration and programmatic storytelling.
This symphony, composed in 1830, was groundbreaking in many ways. It tells the story of an artist who falls into a deep obsession with a woman and experiences a series of hallucinations and dreams.
The symphony showcases Berlioz's innovative use of orchestration and programmatic storytelling, making it a milestone in the Romantic era of music.