General Synonyms:
* Head
* Leader
* Boss
* Director
* Manager
* Principal
* President
* Head honcho
* Top dog
* Main man/woman
* Supreme
More Specific Synonyms:
* Commander (for military leaders)
* CEO (for business leaders)
* President (for government leaders)
* Chairman (for board leaders)
* Captain (for ship leaders)
* Superintendent (for educational leaders)
Informal Synonyms:
* Big cheese
* Bigwig
* Top brass
* Main squeeze
The best synonym will depend on the specific context and tone of your writing.