What threat is banquo to Macbeth?

No logical reason, but then fear is not a logical thing. Although Macbeth says, "My fears in Banquo stick deep," he is not totally clear about why he is afraid of him.

Most likely, he is afraid that Banquo knows too much about him, knows what the witches said to him and how troubled he was by it, and as a result may suspect Macbeth of murdering Duncan.

He is also annoyed by the prophecy of the "fruitless crown" and the prophecy that Banquo will be the father to kings. To prevent this, Macbeth would have to kill not only Fleance but also Banquo, to prevent Banquo from fathering more children. This prophecy does not threaten Macbeth, yet he feels threatened by it. All of this makes the ball of anxiety which he feels because of the murder focus irrationally on Banquo.