Taste:
* Sour: This is the most direct opposite of sweet in terms of taste.
* Bitter: Also a contrasting taste, but with a more complex, sometimes unpleasant flavor.
* Salty: While not directly opposed, salty often contrasts with sweet in culinary contexts.
Other meanings:
* Bitter: This word can also be used metaphorically to describe something unpleasant, harsh, or painful, which is the opposite of something sweet or pleasant.
* Sour: Can also be used metaphorically to describe something unpleasant, negative, or spoiled.
* Unsweet: This simply means not sweet, and can be a neutral opposite.
* Unpleasant: This is a general opposite of pleasant, which can include sweet experiences.
The best word to use depends on what you're trying to convey.