opposite of 'sweet'

No. It's sour.

I'd say it could be either "sour" or "bitter" but "sour" is more common.

example: sweet and sour soup

If you are using the term sweet to describe a taste sensation bitter or sour aren't necessarily opposites of sweet. In this case the opposite of sweet might more appropriately be labeled dry as in dry wine or dry beer.

However, if you're using the term sweet as in "sweet victory," then the opposite would be bitter as in "bitter defeat."