Simone Weil because she was just so unflinchingly (and sometimes stupidly) good. Unlike most philosophers, she put her money where her mouth was and put herself in miserable and dangerous situations to do what she thought was right. She was no armchair philosopher by any stretch.
As for how much she accomplished and how much I agree with her ideas, these is far less impressive than her aforementioned moral character, bizarre resume, and tragic death. As an atheist who does not agree with anarchism, many of Weil's ideas challenge my own quite aggressively. That said, I really think she would have been an improvement upon the drivel that many of her contemporaries served up. If it takes a crazy, self-hating Jewish Catholic with an eating disorder to take down the pedophile Michel Foucault and his ilk, so be it.
Her brief exchange with Simone de Beauvoir always cracks me up:
'In Memoirs of a Dutiful Daughter, de Beauvoir reports her first and perhaps only personal interaction with Weil in, most likely, 1929. "A great famine had just begun to devastate China," she writes, and:
I was told on hearing the news she [Weil] had wept; these tears commanded my respect even more than her philosophical talents. I envied her for having a heart that could beat right across the world. One day I managed to approach her. I don't remember how the conversation began; she declared in no uncertain terms that one thing alone mattered in the world today: the Revolution that would feed all the people on earth. I retorted, no less peremptorily, that the problem was not to make men happy, but to find a meaning for their existence. She looked me up and down: "It is easy to see you have never gone hungry," she said. Our relationship stopped there. (239)'