I would also have to say Micheal Moore since he has created very controversial documentaries that really highlight the major issues in our country, but he throws it in your face.  Also John Stewart is a man the truly interviews his guests.  If John has on a conservative right winger he is not afraid to ask him/her very tough questions.  For example he confronted Mike Huckabee about gay marriage and asked Mike to give him a logical reason why gay people cannot marry...he nailed him!

There are many possible people who fit into this category. Martin Luther King,Jr., Caesar Chavez, and even Ralph Nadar immediately come to mind. Ralph Nadar is still alive and his questions still make people uncomfortable. As a consumer advocate for many years, he has challenged big business over the way they ignore consumer's safety and for their focus on profits instead of production of quality products. Nadar wrote a landmark book in 1965 called "Unsafe at Any Speed". He accused the automobile industry of ignoring safety standards when producing their products. In 1969 he founded the Center for the Study of Responsive law whose purpose is to expose corporate safety violations and government inaction. He has run for president several times and some say he was responsible for Al Gore losing the presidential election in 2000. He makes many people uncomfortable with his questions over corporate greed and yet his work has lead to important reforms.

I think a lot of today's political commentators and documentary makers do just this.  As an example, Michael Moore's documentaries (such as "Fahrenheit 9/11") have received a lot of criticism, but they have also inspired many discussions.  The same can be said for Morgan Spurlock's work (such as "Supersize Me").  These men, and others like them, use the medium of film to get their message out to people and to get people talking.  The controversial nature of their material makes people talk, whether they agree or disagree with the filmmakers.

One of the greatest philosphers of the 20th century was Ayn Rand. She spent her life making people uncomfortable. Rand, who wrote "The Fountainhead," and "Atlas Shrugged," was almost completly responsible for the Objectivist theory.

"Rand advocated an ethics of rational self-interest. The hero of her best-selling Atlas Shrugged states, “I swear—by my life and my love of it—that I will never live for the sake of another man, nor ask another man to live for mine.” The moral purpose of anyone’s life is his or her own happiness; he or she exists to serve no other individual or group. The moral standard by which one guides one’s actions is set by the objective requirements of human life. Thus, Rand rejected two common theses in ethical theory: that selfless sacrifice is moral and that acting in one’s self-interest means doing whatever one feels like. She rejected as “moral cannibalism” any form of altruism—that is, any claim that the selfless sacrifice of some humans for the benefit of others is moral. She also argued that, since feelings are not tools of cognition, they are not reliable guides to action; hence, one must rationally define the principles of action that will allow one to achieve the values necessary to sustain one’s life."

Many people fit this description, but since you asked for just one, I'll add Ben Stein to those already mentioned.  Hist latest film, Expelled: No Intelligence Allowed, presented a view of academia in our world that clearly "challenged" many people judging by the hostility it generated (in this sense, he has something in common with Michael Moore).  Ifyou haven't seen the movie yet, rent it and see what you think.

Margaret Sanger challenged American society during the early twentieth century, by suggesting women become educated about birth control. She advocated birth control especially for the immigrant poor living in the lower east side of Manhattan. The term 'birth control' was as uncomfortable as it got during the turn of the century. She established The American Birth Control League which is now known as Planned Parenthood. Sanger's beliefs and actions violated many obscenity laws in New York City and as a result she found herself at odds with the law and from time to time in jail. However, Sanger believed that the ills of the immigrant poor in New York City were compounded by the pregnancies women were having. It was not uncommon for an Irish immigrant woman living in the lower east side to have 12 to 14 pregnancies. Depending upon how many of the children survived infancy, 12 to 14 was just more than these folks could support. Most surviving children although perhaps loved were undernourished, and grew up uneducated. Sanger faced a triple edged sword, for she challenged society, the law, and religious doctrine when she asked the difficult questions.