It is hard to imagine coming to a definitive answer to this question, except to say for the purposes of this venue that Macbeth and Lady Macbeth could be the two greatest candidates. We should first consider the title of the play and that the easy complacency of simply attributing the tragedy to the husband, rather than his wife, could limit the analysis. That is, Lady Macbeth has a claim on being the tragic figure as does the Thane of Glamis.
The reason we still discuss these issues even 400 years later is that Shakespeare has woven an enchanting plot with mastetful usage of irony and foreshadowing. Macbeth's early "fair and foul day" observation seems to indicate that he possess some early knowledge that this moment in his life will be at least morally ambiguous. Well before Duncan's murder, even the witches chant, upon Macbeth's approach, that "something wicked this way comes."
However it is only after Lady Macbeth instructs Macbeth to abandon any warm, humane feelings, especially for King Duncan, that would prevent an ambitious murder. The ultimate questions are: Was the murderous instinct always within Macbeth's constitution? Did Macbeth only find the resolve to murder after being enticed by the witches? Or was it finally Lady Macbeth's own spellbinding tutelage on how one becomes a doer of regicide which is to be considered?