Formal:
* Magnificent: Impressively beautiful, elaborate, or grand.
* Splendid: Very impressive and beautiful.
* Exquisite: Extremely beautiful and delicate.
* Superb: Of the highest quality; excellent.
* Extraordinary: Uncommon; remarkable.
Informal:
* Awesome: Extremely impressive; inspiring awe.
* Fantastic: Extremely good; wonderful.
* Terrific: Excellent; wonderful.
* Brilliant: Very impressive; exceptionally clever or skillful.
* Fabulous: Extremely good; wonderful.
Other options:
* Delightful: Giving great pleasure.
* Marvellous: Extremely good; wonderful.
* Exceptional: Unusually good; outstanding.
* Remarkable: Worthy of attention or notice.
* Impressive: Making a strong impact; memorable.
The best word to use will depend on the context and the specific meaning you want to convey.