You can say: He/she is very efficient at their job OR works to the best of their ability

He is a hard worker, means that he works hard and is not lazy. Diligent means that they are not only hard working, but this expression may imply that they also are careful to fulfill all the assigned tasks, or are attentive to detail. When you say a person makes efforts, that means they try hard. But that sometimes may imply that they try even if they are not that good at it. So a student with good motivation and good effort may get some marks for their effort, even though the final result may not be that good. But sometimes this expression is used in a very positive way also.

Both are used, but most people just say hard worker.

Pretty common to say "He/she is a hard worker"