General opposites:
* Unsuccessful: This is the most straightforward opposite.
* Failed: This implies a complete lack of success.
* Ineffective: This focuses on the lack of desired results.
* Fruitless: This suggests that no positive outcome was achieved.
* Futile: Similar to fruitless, but emphasizes the pointlessness of the effort.
Contextual opposites:
* Failing: This focuses on the current state of being unsuccessful.
* Unproductive: This emphasizes the lack of output or progress.
* Inefficient: This focuses on the lack of optimization in achieving goals.
* Disappointing: This focuses on the emotional impact of lack of success.
Other options:
* Unfulfilled: This focuses on the lack of satisfaction or achievement.
* Mediocre: This suggests a lack of excellence or distinction.
The best opposite for "successful" depends on the specific context and the nuance you want to convey.