General:
* Excellence
* Superiority
* Magnificence
* Splendor
* Eminence
* Glory
* Sublimity
* Wonder
* Majesty
* Immensity
* Extraordinary
* Exceptional
* Remarkable
* Outstanding
* Distinguished
* Illustrious
* Notable
* Renowned
* Famous
* Legendary
* Heroic
* Grandiose
For specific situations:
* For a person:
* Genius
* Masterpiece
* Icon
* Legend
* Titan
* Giant
* Luminary
* Pioneer
* Visionary
* Leader
* For an achievement:
* Triumph
* Masterwork
* Milestone
* Landmark
* Breakthrough
* For a place:
* Wonder
* Paradise
* Haven
* Mecca
You might also use figurative language to convey greatness:
* Towering
* Soaring
* Unsurpassed
* Unparalleled
* Unrivaled
The best synonym to use will depend on the specific meaning you want to convey.