Answer:
Selflessness.
According to Defoe, a servant's best quality is selflessness. The ideal of a servant in Robinson Crusoe is Friday. Friday is an indigenous person from the Caribbean. He is rescued by Crusoe, and becomes his servant. Friday is a key figure in the novel, and an example of the selflessness Defoe values. Friday is willing to give anything for Crusoe, and he even asks to be killed rather than being forced to leave the service of his master.