The best description of Banquo is given by Macbeth himself in Act 3, Scene 1, when he is expressing his fears and intentions in a soliloquy just before talking to the two murderers.
Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be feared. 'Tis much he dares,
And to that dauntless temper of his mind
He hath a wisdom that doth guide his valor
To act in safety.
Macbeth goes on to compare himself and Banquo with Marc Antony and Octavius Caesar. Antony felt inferior to Caesar, as Macbeth feels inferior to Banquo. Macbeth knows that Banquo can see right through him and will always be his enemy and his nemesis, even though he is treating Macbeth with formal deference as the new monarch. Banquo was with him when they met the three witches. He knows they promised Macbeth he would be Thane of Glamis and King of Scotland, and he feels certain that Macbeth murdered Duncan in order to achieve his ambition and fulfill the witches' prophecies. Macbeth says of Banquo:
There is none but he
Whose being I do fear...
While Macbeth is expressing his fear and hatred of Banquo in this soliloquy, he is also directly and indirectly characterizing him as a wise, patient, highly intelligent, honorable and just man who is more worthy than Macbeth to be the sire of a whole line of Scottish kings, as the witches predicted.