General Negatives:
* Terrible
* Awful
* Dreadful
* Horrible
* Appalling
* Disappointing
Specific to certain aspects of "amazing":
* Mediocre (if "amazing" means exceptionally good)
* Ordinary (if "amazing" means extraordinary)
* Boring (if "amazing" means exciting)
* Unremarkable (if "amazing" means notable)
* Unimpressive (if "amazing" means impressive)
Depending on the context, you could also use:
* Bad
* Poor
* Weak
* Uninteresting
* Forgettable
* Dull
Ultimately, the best opposite for "amazing" will depend on the specific meaning you want to convey.