Formal:
* Preeminent: Highly distinguished or outstanding; superior
* Preeminent: Standing above others in importance, rank, or achievement.
* Paramount: Of chief importance; supreme
* Principal: Most important or significant
* Primary: First in order of importance; main
* Chief: Most important; principal
* Prime: Of the highest quality or importance
* Premier: First in rank or importance
Informal:
* Top: The highest or most important position
* Leading: Most prominent or influential
* Top-notch: Of the highest quality or standard
* Best: The most excellent or desirable
* Number one: The most important or successful
The best word to use will depend on the context of your writing.