Why was Haydn "the father of the symphony"?
Joseph Haydn is frequently called "the father of the symphony." He made important advances in symphonic composition, but he wasn't the first to write symphonies in the classical form.
The title arguably should go to Johann Stamitz, an important composer of the Mannheim school. During the Baroque era, there were compositions called "sinfonia," but they weren't symphonies in the classical-era sense. The closest Baroque equivalent was the concerto grosso.
A Stamitz symphony used an orchestra similar to what Haydn and Mozart employed. Most of his symphonies were four-movement works, in the familiar pattern of a fast movement, a slow one, a minuet, and a very fast movement.
Haydn's biggest contribution to symphonic form was the full elaboration of first-movement sonata form, with an exposition, development, and recapitulation and often a slow introduction. He enlarged the form, with symphonies running to half an hour in length. Earlier symphonies seldom went much over fifteen minutes. The best of his symphonies are far more interesting and memorable than any by his predecessors.
Later composers continued to change the form. Beethoven used a scherzo in place of the minuet and added voices to one symphony. Later composers went beyond four movements, linked the movements, and created symphonies close to two hours in length.
In light of these considerations, how strong is Haydn's claim to being the father of the symphony? Does J. Stamitz have a better claim? Should we just say there was no one person who brought the symphony as we now think of it into existence?