Your question has been argued for centuries, so my answer is certainly not absolute. What I will do is offer some evidence on both sides of the argument.
In favor of Macbeth’s being the more ambitious of the two, some critics point out that Macbeth’s “murderous resolve” increases as the play progresses, while Lady Macbeth weakens to the point of insanity and suicide. On the other hand, it is not until Lady Macbeth taunts her husband into going through with the murder of Duncan that Macbeth overcomes his reluctance. It is Lady Macbeth who decides on the time, place, and method of killing Duncan.
My opinion is that the overweening ambition that originates with Lady Macbeth infects her husband. In the end, it is Macbeth who commits the murders and faces his own end with more courage than his wife faced hers.