General synonyms:
* Hardworking
* Industrious
* Assiduous
* Conscientious
* Meticulous
* Careful
* Thorough
* Dedicated
* Persevering
* Persistent
Synonyms emphasizing effort:
* Laborious
* Tireless
* Unflagging
* Relentless
Synonyms emphasizing attention to detail:
* Scrupulous
* Painstaking
* Exacting
* Rigorous
Synonyms emphasizing focus on a task:
* Focused
* Attentive
* Concentrated
More formal synonyms:
* Sedulous
* Incessant
* Untiring
The best synonym to use will depend on the specific context and the nuance you want to convey.