Macbeth does the physical murdering of King Duncan, but Lady Macbeth, it can be said, succeeded in convincing him that he must murder the King. Indirectly, the witches are also responsible for the murder.
It is after the prophecy that the witches give Macbeth about his succession first to Thane of Cawdor and then King that ignite his secret ambition, wanting to be king. So when the witches give Macbeth the prophecy, they expect murder and mayhem.
Therefore, the witches, Lady Macbeth and Macbeth are all responsible for the murder of King Duncan.
Macbeth is most immediately responsible for Duncan's death, and so bears the most guilt. Lady Macbeth conspired, and feels responsible, so she is also guilty. If you wanted to argue that the witches should be held distantly guilty, I'd go with that…though they didn't force any specific action.
Macbeth and his wife, Lady Macbeth, are equally guilty for King Duncan's death.
I think Lady Macbeth.
I think that Macbeth was forced into it, more likely tricked, by Lady Macbeth. Even though Macbeth killed Duncan, Lady Macbeth planned it.