General synonyms:
* Successful
* Efficient
* Productive
* Powerful
* Potent
* Fruitful
* Operative
* Functional
* Working
* Impactful
More specific synonyms:
* For actions: Achievable, attainable, feasible, workable, practicable
* For communication: Persuasive, convincing, compelling, cogent
* For medicine: Therapeutic, curative, remedial
* For teaching: Instructive, educational, enlightening
Antonyms:
* Ineffective
* Useless
* Futile
* Unproductive
The best synonym will depend on the specific meaning of "effective" in your context. Consider what nuance you want to convey and choose the synonym that best fits.