Der Blaue Reiter
Kandinsky, three of whose paintings hang perennially on the right side of the blog, is, arguably, the most important artist of the 20th century. There are a number of reasons why: he was a strong participant in two important Expressionist movements: Der Blaue Reiter, as a founder (see the namesake of the movement above) and the Bauhaus School, as a teacher; and he is credited with producing the first abstract work in the art world (the term, Abstract Expressionism, was coined in reference to Kandinsky). And all of this commentary does not even begin to cover the amount of changes Kandinsky underwent within each style, always with an amazing sense of color and composition.
The painting above, Composition VII, is considered Kandinsky's magnum opus; he said that he put the most work of any of his paintings into that piece.
Glass Painting with the Sun