It is Telemann by some margin.
Strange that he was considered superior to Bach in his day. How tastes change.
Telemann (1681 - 1767) was only four years older than Bach, and yet, by the time Bach applied to the post of Kantor in Leipzig, Telemann was already the most famous musician in Germany while Bach was still considered an upstart. Nevertheless both composers were good friends (Telemann was godfather to Carl Phillip Emanuel).
Yet by 1799, J. S. Bach was already the centre of the "Sun of Composers" , and Telemann had already been relegated to one of the rays.
Christoph Wolff ("J. S. Bach: The Learned Musician" - OUP) has a very interesting theory. Quickly forgotten composers like Rameau, Telemann, Scarlatti, never engaged in teaching to the extent that Bach did. He suggests that it was Bach students and the fact that Bach was associated with an academic centre that guaranteed his survival (the excellenc of his music is taken for granted).
Another interesting fact: Telemann was actually a lawyer, and he taught himself music and composition while studying law.
Best wishes,
Bernhard.