There has been a useful discussion thread on this issue as detailed below. Macbeth is most certainly the focus of the witches' as they make reference to meeting him in the eighth line of the play. However, Banquo is also challenged by their ambiguous and engaging prophesies. It is what each character chooses to do with the temptations they are faced with which outlines their fate.
Lady Macbeth calls on the spirits and evil forces to support her in the plan to kill Duncan. She too pays for her evil desires.
There's ample evidence that the witches are after both Macbeth and Lady Macbeth. The "rump fed runyon" they refer to in Act 1, scene 3 is no doubt Lady Macbeth (by the way, a ronyon is a mangy or scabby creature, which shows how little respect they have for her):
A sailor's wife had chestnuts in her lap,
And munch'd, and munch'd, and munch'd:--
'Give me,' quoth I:
'Aroint thee, witch!' the rump-fed ronyon cries.
Her husband's to Aleppo gone, master o' the Tiger:
But in a sieve I'll thither sail,
And, like a rat without a tail,
I'll do, I'll do, and I'll do.
And, as was mentioned above by kiwi, in scene 5 Lady Macbeth summons the witches powers:
Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me from the crown to the toe top-full
Of direst cruelty! make thick my blood;
Stop up the access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
The effect and it! Come to my woman's breasts,
And take my milk for gall, you murdering ministers,
Wherever in your sightless substances
You wait on nature's mischief! Come, thick night,
And pall thee in the dunnest smoke of hell,
That my keen knife see not the wound it makes,
Nor heaven peep through the blanket of the dark,
To cry 'Hold, hold!'
Clearly the Lady is possessed and wants more. She asks for the witches' aid for the dark deed she is already planning. And the witches know well what they are about. Lady Macbeth and Macbeth are a deadly duo; acting alone, neither could kill King Duncan. They need and reinforce each other and act in concert to plan and carry out the murder.