In Shakespeare's Macbeth, while the witches put the idea of being king into Macbeth's head, and Lady Macbeth manipulates him into going through with his first killing, I agree with the previous poster that in this case, the obvious choice for most influential character is the best.
Macbeth takes the prediction that he will be king and turns that into he will assassinate Duncan. The witches don't say anything about that. And while Lady Macbeth talks him into doing his first killing, he orders the rest completely on his own. He shuts his wife out of the decision-making process and takes over the planning himself. He also goes to the witches in Act 4: they don't come to him.
The most influential person in the play, by far, is Macbeth.
I guess the obvious answer would be Macbeth himself. He is the one who does all the most important things in the play (or, at times, orders others to do them).
I suppose you could also say that either the witches or Lady Macbeth are the most influential. Both the witches and Lady Macbeth have a lot to do with Macbeth doing the things he does. So they influence him and he influences the play.
So it's kind of up to you to decide. Who is more influential -- the guy who does stuff, or the people who sort of help influence him to do the stuff?
It could have been Lady Macbeth
It has been repeatedly told that Macbeth was the most misogynist Shakespearean play, partly because of the implication of evilness lain in the women involved, partly because of the indecise character of Lady Macbeth, proven weak afterwards by her insanity and consequent untimely death. Although Freud tried to analyze Lady Macbeth’s behaviour on the grounds of an incomplete womanhood due to her childlessness, her movement along the original text seems somehow to lack consistency, the very few hints at her humanity being, from my personal point of view, insufficient to raise such a level of guilt. At first Lady Macbeth even seems superior to her husband, who, although proven courageous in battle, depicts a cowardly lack of determination. Moreover, she seems to be the one who assumes the role of the leader, whereas her husband only submits to her homicidal requests, more or less against his own will, taking the unflattering part of an agent of the woman’s dark desires. Starting as a full member of Shakespeare’s gallery of great feminine characters, seemingly filling in the missing part of the “villainesse”, Lady Macbeth gradually loses importance (within the plot) and mental sanity, to the point of a death suggested as undignifying. Her downfall parallels Macbeth’s ascension towards a status of a tragic hero who made the fatal mistake. In some sense, her destiny mirrors Katherina’s, the shrew – she is “tamed”, reduced, silenced by a playwright who can be conveniently interpreted as a misogynist from this point of view.
Do you have any doubt in this matter? I don't have any. It must be Macbeth whose tragic disaster illustrates the complex problematic of evil. The Witches appear at the outset of the play to declare their intention to meet Macbeth on the heath, and the proposed meeting in act1 sc.3 reveals the battle between ambition and conscience in Macbeth, the same Macbeth who has earlier been characterized in absentia as 'Bellona's bridegroom' and 'valour's minion'.
Macbeth's killing of Duncan and his usurpation of the throne, followed by the killing of Banquo and the brutal massacre of the family of Macduff lead the play to its climax.It is Macbeth who chooses to meet the Witches to know more about his future, and that marks the beginning of his final doom.
Yes, Lady Macbeth is also a very remarkable character; but what she does or what happens to her is intricately connected with the 'vaulting ambition' in Macbeth.She kills herself in utter despair and anguish ahead of Macbeth's defeat and death. Macbeth goes down fighting to the last.