Definitely for the first half of the play Banquo and Macbeth are presented as opposite figures, despite obvious similarities. Both of them are good fighters and have proven their valour for the king and both are the subject of prophecies made by the witches.
However, in the play he is morally superior to Macbeth and is clearly not unhappy in the way that Macbeth is haunted and tortured by the prophecies and what is happening to him. Banquo differs in his reaction to the witches and their prophecies: he is sceptical of what they predict and sees them as "the devil" and "instruments of darkness". Also, although he clearly does struggle with ambition, he prays to God for help in controlling them. Banquo is a rebuke to Macbeth's decision to knowingly embrace temptation and give in to ambition and also represents a challenge to his desire to rule, for it is Banquo's descendants who will rule, and not Macbeth's descendants. It is therefore fitting that it is the ghost of Banquo that haunts Macbeth rather than the ghost of Duncan.
So although Banquo is not "good" - he clearly suffers from the same temptations as Macbeth - the difference is that he, unlike Macbeth, resists and does not give in to this temptation.