What registry key is considered to be the most important key, containing hardware, software, and security data?

HKEY_LOCAL_MACHINE (HKLM):
The registry key that is the most important key and contains hardware, software, and security data. The data is taken from four hives: the SAM hive, the Security hive, the Software hive, and the System hive. In addition, the HARDWARE subkey of HKLM is built when the registry is first loaded, based on data collected about the current hardware configuration.