This depends on if you're referring to the most common type of fiction to read for leisure/pleasure, or the most common type of fiction that is assigned in school.
If we are talking about reading for leisure and pleasure, the most common type of fiction to read would be within the categories of suspense/thriller/mystery. Think John Grisham, James Patterson, Dean Koontz, etc.
If, on the other hand, you are referring to the type of fiction most often assigned for academic reading, it would be literary fiction. It's a little difficult to define literary fiction, but it is pretty much the opposite of any type of "genre fiction" in that it does not fall into a specific genre or category, simply because it does not rely so heavily on its plot structure alone. Rather, literary fiction is more likely to be along the lines of realistic fiction, and relying heavily on the thoughts of characters, thematic ideas, symbols, rich language with literary devices such as figurative language, imagery, and complex characterization.