what makes Duncan a good king?

Duncan was generous to his friends,

He gives Macbeth the Thane of Cawdor title "go pronounce his (Cawdor's) present death and with his former title greet Macbeth"

He Tells Macbeth that he is really going to go places, "I have begun to plant thee, and will labor To make thee full of growing."

Duncan alludes to rewarding Banquo as well, "Noble Banquo, That hast no less deserved, nor must be known No less to have done so; let me enfold thee And hold thee to my heart."

Duncan, seems to be a "good king" but he is also somewhat naïve and trusting in his appraisal of his friends.