* Αποτελεσματικός (apotèlesmatikós): This translates most closely to "effective" or "result-oriented." It emphasizes achieving the desired outcome. This is often a good choice if you're talking about efficiency in terms of productivity.
* Αποδοτικός (apodotikós): This means "productive" or "high-yielding." It focuses on the output relative to the input. This is a good choice when discussing economic or industrial efficiency.
* Οικονομικός (oikonomikós): This means "economic" or "thrifty." It emphasizes minimizing waste and resources. This is suitable when describing something as efficient in terms of cost or resource usage.
* Γρήγορος (grígoros): This simply means "fast" or "quick." While not a direct translation of "efficient," it can be used if the context emphasizes speed as the primary aspect of efficiency.
To choose the best word, consider what aspect of "efficiency" you want to highlight. For example:
* "An efficient worker" could be ένας αποτελεσματικός εργαζόμενος (énas apotèlesmatikós ergazómenos) or ένας αποδοτικός εργαζόμενος (énas apodotikós ergazómenos).
* "An efficient system" could be ένα αποτελεσματικό σύστημα (éna apotèlesmatikó sýsthma) or ένα οικονομικό σύστημα (éna oikonomikó sýsthma).
Therefore, there's no single answer. Consider the context to select the most appropriate word.