Direct Opposites:
* Awful
* Terrible
* Horrible
* Dreadful
Less Direct but Still Opposite in Meaning:
* Mediocre
* Ordinary
* Boring
* Unremarkable
* Disappointing
* Abysmal
* Poor
* Shocking (in a bad way)
The best word to use depends on the specific context and the nuance you want to convey.