Which composer is most closely linked to the development of the string quartet genre?

The composer most closely linked to the development of the string quartet genre is Haydn. Joseph Haydn is often referred to as the "father of the string quartet" because he played a crucial role in establishing the form and structure that would characterize the genre. His contributions include innovative approaches to harmony, counterpoint, and thematic development within the quartet framework.

Haydn wrote a significant number of quartets, ultimately helping to define the conventions of the format, including the use of four distinct voices – two violins, a viola, and a cello – and the establishment of the four-movement structure that became standard. His works set a stylistic template that later composers like Mozart and Beethoven would build upon. Thus, while other composers made significant contributions to chamber music, it is Haydn who is most notably recognized for shaping the string quartet into a major musical form.