For a person in a high position:
* Leading: The leading figure in the industry.
* Top: The top executive of the company.
* Chief: The chief of staff.
* Head: The head of the department.
* Principal: The principal of the school.
* First: The first minister of the country.
* Foremost: The foremost expert in the field.
For something of the highest quality:
* Best: The best restaurant in town.
* First-rate: A first-rate performance.
* Superior: A superior product.
* Exceptional: An exceptional talent.
* Outstanding: An outstanding achievement.
For something that is first or most important:
* Initial: The initial meeting.
* Primary: The primary goal.
* Main: The main event.
* Topmost: The topmost priority.
* Leading: The leading candidate.
Other options:
* Inaugural: The inaugural performance.
* Opening: The opening night.
* Opening: The opening ceremony.
To choose the best replacement, consider the specific meaning you want to convey.