This doesn't appear to be a subject-related question so I'll chime in. In my area of Combinatorics and Graph Theory, a part of Discrete Mathematics, without a doubt, the answer would be Paul Erdős. Truly revolutionary!
In the area of Combinatorial Dynamics the answer would be Oleksandr Mykolayovych Sharkovsky who proved in 1964 his famous theorem and thus started a new field of research in Dynamical Systems.
While his "Lectures on Physics" (see http://www.feynmanlectures.caltech.edu ) are not intended for novices, they are famous for his way to approach the topics.