I do think Malcolm would make a good king. Although he is young, he is unusually clever and level-headed. He leaves the kingdom to gather an army when Macbeth kills his father, and then he tempts Macduff by suggesting that he would make a horrible king, in order to see if he can really trust Macduff. He decides he can. He is a good judge of character. He also supports Macduff when his family is killed, and tells him to turn his grief into revenge.
Shakespeare takes care to mark Malcolm with the qualities a successful ruler requires. His first response to the news of his father King Duncan's murder is to demand who has done it, showing he is self-possessed in moments of extreme stress (II, iii). He immediately realizes that the plan is to cast suspicion on him and his brother Donalbain, and agrees with the latter's plan to flee for safety. He goes to England, the best place to gather aid to resist Macbeth. Joined there by Macduff, he is not so foolish as to trust the latter immediately, but instead tests his character by reciting an imaginary litany of sins (IV, iii). He realizes, as Macduff has not, the danger Macduff's family has been placed in by his sudden flight, and when the news of their murder arrives, first gives the practial advice to Macduff to express his feelings, and then turns his grief in the direction of anger and revenge (IV, iii). Finally, at the end of the play, we see him in a similar situation with old Sewell, whose son has died, and generally arranging matters, bringing the violence of the Macbeth era to a close with his own enthronement:
...this, and what needful else
That calls upon us, by the grace of Grace,
We will perform in measure, time, and place:
So, thanks to all at once, and to each one,
Whom we invite to see us crown'd at Scone. (V, viii)
As befits his role in restoring the order shattered by his father's death, he has the final words in the play.