Instead of thinking first about the qualities of Banquo it would be well worth your while to think a bit wider in terms of Banquo's overall function in the play, and from that point go back and pick out his characteristics.
In this play one of the main purposes of Banquo's character is to act as a contrast to the character of Macbeth. Banquo is brave and noble - characteristics that Macbeth arguably doesn't possess. Interestingly, like Macbeth, Banquo is ambitious, but signficantly, unlike Macbeth, Banquo does not act on those ambitious thoughts to convert them into action. Indeed, Banquo has the presence of mind or the ability to question the weird sisters and their prophecies: "oftentimes, to win us to their harm, / The instruments of darkness tell us truths, / Win us with honest trifles, to betray us." This ability to reflect on the prophecies is a quality that Macbeth definitely does not possess.
If you think about this comparison a bit further it is clear that Banquo's character stands in the play for a path that Macbeth did not take, and acts as a reminder that ambition by itself does not necessarily have to be translated into treachery and assassination. We can see therefore why it is Banquo's ghost (and not the ghost of Duncan) that haunts Macbeth and why this haunting is so powerful. The ghost interestingly reminds Macbeth that Banquo did not copy Macbeth's response to the witches' prophecy.