Banquo is immediately suspicious of the witches, and, though he believes the veracity of the prophecies, he's suspicious of the prophets:
But ’tis strange;
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence—
Macbeth actually expresses similar doubts to start with, but soon seems to work himself round that the crown really is his to gain:
This supernatural soliciting
Cannot be ill, cannot be good. If ill,
Why hath it given me earnest of success,
Commencing in a truth?
That's their immediate reactions.
Yet when we meet Banquo later in the play, he is thinking that maybe he could get something out of the witches' prophecies:
If there come truth from them—
As upon thee, Macbeth, their speeches shine—
Why, by the verities on thee made good,
May they not be my oracles as well
And set me up in hope?
So Banquo is - potentially - leaning in Macbeth's direction, though twenty trenched gashes to the head soon puts a stop to that particular line of character development. Macbeth does actually come to doubt the witches, eventually, but right at the end:
I pull in resolution and begin
To doubt the equivocation of the fiend
That lies like truth.
Eventually, he comes to see that the witches' prophecies are lies which seem like truth. But, too little, too late.