You really need to think about what your answer to this communicates. If you answer MLK, Desmond Tutu, and Mother Theresa, you have communicated something to your interviewer that's different than if you answer Paul Samuelson, Milton Friedman, and Paul Krugman.
It's a very clever question. The candidate is tricked into thinking that his knowledge is being tested, when the interviewer is really trying to figure out what the candidate thinks is important in life.