If you're talking about something unpleasant or bad:
* Wonderful
* Amazing
* Fantastic
* Splendid
* Excellent
* Delightful
* Fabulous
* Great
* Superb
* Terrific
* Outstanding
* Magnificent
* Perfect
* Brilliant
If you're talking about something that inspires awe or wonder:
* Awe-inspiring
* Sublime
* Majestic
* Grand
* Impressive
If you're talking about something that is extremely good or skilled:
* Exceptional
* Outstanding
* Masterful
* Proficient
The best opposite word for "awful" will depend on the specific situation and the intended meaning.