It's impossible to say definitively which inorganic compound is the most important as it depends on the context and what you're considering "important."

However, some strong contenders for the title include:

* Water (H₂O): Arguably the most essential compound for life. It plays a crucial role in biological processes, serves as a solvent, and regulates temperature.

* Oxygen (O₂): Essential for respiration and energy production in most living organisms.

* Carbon Dioxide (CO₂): A key component of photosynthesis, the process plants use to make food. It also plays a role in regulating the Earth's climate.

* Sodium Chloride (NaCl) (Table Salt): Important for maintaining electrolyte balance in the body, essential for nerve and muscle function.

* Nitrogen (N₂): Makes up about 78% of the Earth's atmosphere and is a crucial component of DNA, RNA, and proteins.

* Calcium Carbonate (CaCO₃): The main component of seashells, bones, and coral reefs. It also plays a vital role in the carbon cycle.

Ultimately, the "most important" inorganic compound depends on your perspective and the specific context you are considering.