Duncan was the legitimate king. His subjects liked him and willingly obeyed him. Macbeth was forced to rule by terror because his subjects did not regard him as legitimate and suspected him of murdering Duncan and Banquo, although their were afraid to discuss such matters even with their closest friends. Macbeth describes Duncan as follows:
Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels, trumpet-tongued, against
The deep damnation of his taking-off... (1.7.16-20)
Then in Act 5, Scene 2, lines 19-25) Angus says of the beleaguered Macbeth:
Now does he feel
His secret murders sticking on his hands.
Now minutely revolts upbraid his faith-breach.
Those he commands move only in command,
Nothing in love. Now does he feel his title
Hang loose about him, like a giant's robe
Upon a dwarfish thief.
Duncan was loved. Macbeth was hated. Duncan was kind and just. Macbeth was forced to rule by terror because he was never accepted as the legitimate monarch and his subordinates were all deserting him. Duncan could retain his crown because the majority of the thanes supported him. Macbeth had to lose the crown because the majority turned against him. Angus calls him "a dwarfish thief."
To understand how Duncan and Macbeth compare as leaders, take a look at how they treat their noblemen. When Macbeth is victorious on the battlefield, for example, King Duncan rewards his bravery by making him Thane of Cawdor. This is significant because it demonstrates Duncan's trust in those around him. In contrast, when Macbeth becomes king, he kills Banquo, his closest and oldest friend. He does this to prevent Banquo's children from becoming kings (as predicted by the witches) and, therefore, as a means of self-preservation. So, Macbeth is less interested in rewarding trust and bravery and more interested in protecting his power.
To contrast Macbeth and Duncan as leaders, consider their attitudes to violence. From the text, it is clear that both men promote the use of violence to solve disputes. In Act I, Scene II, for example, King Duncan's men are involved in a battle to thwart the traitor, Macdonwald. Similarly, in Act V, Scene VIII, Macbeth meets with Macduff on the battlefield as a means of ending their dispute.