General:
* Productive
* Beneficial
* Helpful
* Effective
* Fruitful
* Advantageous
* Constructive
More specific:
* Supportive (if it's about relationships or teamwork)
* Encouraging (if it's about motivation)
* Efficient (if it's about time or resource management)
* Profitable (if it's about financial gain)
The best antonym will depend on the specific context in which "counterproductive" is being used.