Who is the most important thinker in philosophical ethics: Aristotle, Kant, Mill, or Sartre? Why?
Of the four, I would rate Sartre as the least significant as his work only influenced a very small group of thinkers for a short period rather than reframing an entire field. The work of John Stuart Mill has had considerable influence on subsequent political thought and has shaped much of current political thinking about free speech and individual rights, but it has not shifted fundamental underlying concepts, even though it is perhaps the best example of applied consequentialism in the philosophical canon.
Both Kant and Aristotle have reshaped our fundamental understanding of the principles of philosophical ethics, with Aristotle being a monumental figure in virtue ethics and Kant in deontological ethics. Both are equally important, but as Aristotle was considerably earlier, he influenced philosophy for a longer period than did Kant.