General:
* Royal: emphasizing the kingly or queenly nature of the reign
* Sovereign: referring to the supreme power of the ruler
* Long: describes the duration of the reign
* Successful: indicates a reign marked by prosperity and stability
* Turbulent: a reign characterized by unrest and upheaval
* Peaceful: a reign marked by tranquility and harmony
Specific:
* Tyrannical: a reign of oppression and cruelty
* Enlightened: a reign characterized by progress and intellectual advancement
* Golden: a legendary reign of prosperity and happiness
* Iron-fisted: a reign that is forceful and repressive
* Blood-soaked: a reign marked by violence and bloodshed
Figurative:
* Dominant: used metaphorically for something that has a strong influence
* Prevailing: referring to something that is currently in control or power
* Unchallenged: describing something that has no competition or opposition
The best adjective for "reign" will depend on the specific context and the message you want to convey.