Formal:
* Immeasurable
* Boundless
* Infinite
* Unquantifiable
* Incalculable
* Limitless
* Unfathomable
* Uncountable
* Exceeding all bounds
Informal:
* Unbelievable
* Amazing
* Incredible
* Tremendous
* Vast
* Extensive
* Overwhelming
* Extraordinary
The best synonym depends on the specific context and the desired level of formality.