Why is Banquo the only person Macbeth fears?

In Act III, Scene 1, Macbeth expresses his fear of Banquo and his desire to have him murdered.

There is none but heWhose being I do fear; and under himMy genius is rebuked, as it is saidMark Antony's was by Caesar. He chid the sisters,When first they put the name of King upon me,And bade them speak to him; then prophet-likeThey hail'd him father to a line of kings:

Macbeth appears to fear Banquo mainly because the three witches promised Banquo that his offspring would be a whole line of Scottish kings. This obviously gives Banquo a good motive for wanting Macbeth dead. Macbeth can't tell what Banquo is thinking, though. He may be plotting to assassinate Macbeth, or he may be content to wait for Macbeth to die. Banquo makes Macbeth feel inferior, as Octavius Caesar made Mark Antony feel inferior when they shared the rulership of the Roman Empire. Shakespeare records Antony's feelings about Octavius in his play Antony and Cleopatra. Macbeth has become king, but he never really feels like a king. He might feel more self-confident if he had an heir to succeed him on the throne, but he and his wife are childless. Banquo appears to be full of confidence, and he has a son named Fleance who could conceivably become king of Scotland if Macbeth died or was assassinated. Banquo really looks like a king. Macbeth says of him in the same soliloquy quoted above:

Our fears in BanquoStick deep, and in his royalty of natureReigns that which would be fear'd. 

Macbeth hopes to relieve himself of his multiple fears by having both Banquo and Fleance assassinated. In this same scene, he has a meeting with the two murderers he has recruited to kill Banquo and Fleance. It seems significant that Macbeth does not consider killing Banquo himself. This may be at least partly because he is afraid of him.