It is hard, though, to pick the worst:
(1) Jean-Jacques Rousseau, a simpering slug who left a toxic trail of self-pity and self-praise across a landscape of broken lives, beginning with those of his own forsaken children.
(2) Arthur Schopenhauer, "Hitler's favorite philosopher," a venomous frog that spouted poison at the lady who birthed him, then at all women who were not kept slaves.
(3) Simone De Beauvoir, a harpy trained to bring live meat to her "lover" Jean-Paul Sartre, a mother of feminism who seemed to hate women almost as much as Schopenhauer did.
(4) Karl Marx, mad jackel of revolution, yipping poems of the Apocalypse as a young man, then discovering the "dismal science," and weaponizing it against almost everyone.
If I dismiss such "thinkers" before reading more of them than I can without getting a headache, do not call that "ad hominem." A good BS detector is a proper tool in any functioning mind. "By their fruits you will smell them." If your magic works, why don't you use it to make a killing on the stock market? If your take on humanity is so clever, why do you ruin the lives of those you claim to love?
Or is someone even worse than these four?