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.
You could argue in favor of this statement since the witches drive the plot. However, the witches aren't the cause of Macbeth's downfall; they give him the opportunity to succumb to their prophecies, and he takes the bait. Without the witches, Shakespeare would have to find someone else or something else in order for Macbeth to change from a loyal, courageous soldier into a power-hungry murderer. The witches also provide the elements of evil and magic, adding to the mood that supports the plot. Shakespeare puts them at the beginning of the play in order to set the tone. By tempting Macbeth with the possibilities of power, the witches also provide the main conflict Macbeth must deal with throughout the play. I think a very good argument can be made to support your argument.
I would have to argue that point in the sense that MacBeth is the subject of the play - everything revolves around him and his reaction to the idea that the witches put in his head. Yes, the witches are important but as a catalyst - they initiate the action related to the murders. What we are really saying is that every character contributes to the drama. When MacBeth says his mind is 'full of scorpions' for example, he needs someone to say that to - so we can hear how he is feeling. At the banquet, we need to hear everyone's opinions on his behavior when he sees the ghost - we need to hear it from the others that his behavior is unreasonable. Each character is a foil for the others, to compare and contrast.
If you want to argue this, you would have to say that it is the witches that cause Macbeth to do all of the stuff that he does. After all, in terms of actual actions, it is what Macbeth does that actually drives the play. His murders of the various people (Duncan, Banquo, Macduff's family) really make the rest of the action in the play happen.
So you can argue that it is the witches' prophecies that make Macbeth kill those people. Look at what they say and see how each of these murders is at least encouraged by their prophecies.