I don't know if hi's the best in my field, but almost is a very important character in the Psychology's world. Of course, I'm talking about Sigmund Freud, a German doctor that was interested about how mental conditions (psychological states) affect physicaly to the persons. Taking his and others knowledge (from literature, from medical studies, from philosofy...) he started to build the "psychoanalysis theorie". This theorie is based onthe idea that inside every human there are pulses of "creation and love"and pulses of "destruction and death". These pulses are fighting all the time and create tensions that are expresed in some hide expressions like dreams, verbal mistakes and violente reactions.
His idea was very revolutionary in his times because everybody thought that we had a very strong control of ours mental processes and their contained. But nobody suspected that there were hide contents and feelings such as traumas.
He also apport with a new way of healing the persosns: "The healing throw speaking". He realised that if you talk about your problem or if you try to verbalized your traumas or hide thougths, in some way your physical problems (as hysteria) heal.
Now a days is a very popular character, not just in psychology, also in literature, cinema, in medicine and other areas.
I think that he was very openminded and creative. An hyphotesis to explain his geniousity is that he used drugs (it was common between doctors to have drugs and use them) and that they help him to create all his theories.
Well, without Freud we wouldn't be (as occidental culture) the society and the culture that are today. And I wouldn't been studing psychology! hahaha