Formal:
* Excellent
* Outstanding
* Superb
* Magnificent
* Exceptional
* Remarkable
* Splendid
Informal:
* Awesome
* Fantastic
* Brilliant
* Great
* Wonderful
* Amazing
* Fabulous
* Superb
* Smashing (British)
More specific:
* Impressive (for something that evokes admiration)
* Thrilling (for something exciting)
* Delightful (for something pleasant)
* Exhilarating (for something that gives a sense of excitement)
The best synonym for "terrific" will depend on the specific situation and the tone you want to convey.