Paco, and Hanuman - "What genius!" is possible, but it is not the same as "What a genius (he is)!" In "what genius," "genius" refers not to a person (
a genius) but to an abstract quality, like "wisdom" or "beauty." Thomas Edison supposedly said "Genius is 1 percent inspiration, and 99 percent perspiration." (It would be quite different if he had said "
A genius is ...") "Genius" (and also "beauty") is countable when it refers to a person and uncountable when it refers to a quality.