If you're talking about taste:
* Disgusting: This is the most common and direct antonym.
* Bland: This implies a lack of flavor.
* Tasteless: Similar to bland, it describes something with no flavor.
* Revolting: This describes something that is extremely unpleasant to taste.
* Unpalatable: This describes something that is not pleasing to the taste.
If you're talking about something else:
* Awful: This could be used for something that is unpleasant in any way, including taste.
* Terrible: Similar to awful, but emphasizes the negative quality.
* Horrible: Even stronger than terrible, it describes something extremely unpleasant.
The best antonym for "delicious" will depend on the specific situation and what you want to emphasize.