Formal:
* Eminent: This word emphasizes high standing and achievement.
* Distinguished: This word suggests excellence and respect.
* Renowned: This word implies widespread fame and recognition.
Less formal:
* Famous: While less formal, this is still a common synonym for illustrious.
* Notable: This word implies that someone or something is worthy of attention.
* Celebrated: This word suggests that someone or something has been publicly praised.
The best synonym will depend on the specific nuance you want to convey.