Which daughter was most evil and responsible for the King's demise?

First of all, it is quiet clear that the King was responsible for his own demise because of his foolishness. As to which daughter was most evil it is not too clear for both Regan and Goneril were vying for the title. Cordelia on the other hand was far from evil, all she wanted was nothing but good for her father while her other two sisters were more interested in the wealth and could not care less about the King.

From my point of view between Goneril and Regan in terms of the evil done to their father no one exceeded the other; they were equally evil. From the onset instead of being sincere to the King they both resorted to flattering him; telling him just what they knew he wanted to hear just to gain the wealth he had set aside for all three of them. Cordelia in contrast was nothing but sincere, though to the King that was not good enough. He would rather be flattered by her with meaningless words than with the words of the true feelings of her heart. The King was foolish and misjudged which of his daughters to trust and because of they qualities came his downfall.

Goneril and Regan can be seen as equally evil as stated before. While the father was staying with Goneril she told her servants to misbehave, disobey and disrespect him. She could not stand her father being there and could not wait for the day that he would go to Regan's house. Regan defends Goneril when the father complains of
his treatment at the hands of her sister. She tells him, her sister would not do those things he speaks of and that he should not speak of her sister in that way. They both treat him badly and in turn back each other up. They even went as far as to make plans to the effect of securing their wealth and ensuring that the father cannot change his decisions when he divided the kingdom along with other decisions. They did it in a way that if he were to make changes it would be too late. Moreover the King was responsible for his own demise for he was too blind to see the truth and that was the beginning of his downfall. His ignorance, arrogance and misjudgments led to his downfall more than the evil works of Goneril and Regan.