General:
* Essential: This is the most direct and obvious choice.
* Integral: This word implies that the component is necessary for the whole to function properly.
* Indispensable: This word emphasizes that the component is absolutely necessary and cannot be omitted.
* Crucial: Similar to indispensable, this word suggests that the component is vital to the success of the whole.
More specific:
* Fundamental: This word suggests the component is at the core of the system.
* Core: This word emphasizes the component's central role.
* Key: This word suggests that the component is important for unlocking the potential of the system.
* Foundation: This word implies the component provides a base upon which the whole is built.
The best word to use will depend on the specific meaning you want to convey.