General synonyms:
* Diligent
* Industrious
* Assiduous
* Dedicated
* Conscientious
* Persistent
* Tenacious
* Meticulous
More specific synonyms:
* Energetic (emphasizes high energy and effort)
* Zealous (emphasizes passion and enthusiasm)
* Tireless (emphasizes working without getting tired)
* Unflagging (emphasizes never giving up)
* Dutiful (emphasizes a sense of responsibility)
The best synonym to use will depend on the context and the specific meaning you want to convey.