General antonyms:
* Unpleasant
* Disagreeable
* Awful
* Horrible
* Repulsive
* Unwelcome
* Offensive
* Nasty
* Uncomfortable
* Distressing
More specific antonyms:
* Painful (for physical sensations)
* Harsh (for sounds or environments)
* Bitter (for tastes or experiences)
* Depressing (for emotional states)
* Offensive (for interactions)
The best antonym will depend on the context in which you are using the word "pleasant."