General synonyms:
* Ruler
* Monarch
* King (for a male sovereign)
* Queen (for a female sovereign)
* Emperor (for a sovereign ruling an empire)
* Empress (for a female sovereign ruling an empire)
* Head of state
* Supreme authority
Synonyms emphasizing independence and autonomy:
* Independent
* Autonomous
* Self-governing
* Self-ruling
* Free
Synonyms emphasizing power and authority:
* Powerful
* Authoritative
* Supreme
* Ultimate
Synonyms emphasizing the concept of a nation or state:
* Nation
* State
* Country
The best synonym to use will depend on the specific situation and the nuance you wish to convey.