In order to argue that the witches are the most important characters in Shakespeare's Macbeth, I believe you would have to center on a certain issue/theme in the play. You would have to go beyond the fact that they are catalysts or that they put the idea into Macbeth's mind to become king.
You would need to center on the issue of predestination or fate. You would have to center on the idea that the witches really do know the future, and that they, or fate, or God control the future.
The issues of predestination and free will were important in Shakespeare's day, due to the Protestant reformation, and the issue of fate has been debated and studied since at least the Greeks. This is what you would need to center on in order to argue that the witches are the most important characters in the play.
Think of it this way: if the witches know the future, then isn't the future set? If the witches know the future, then what choice does Macbeth have? None, according to the argument you will need to make.
There's a fine line between knowing the future, and determining it.