Final answer:
Joseph Haydn, an esteemed composer, told Leopold Mozart, Wolfgang Amadeus Mozart's father, that Mozart was the greatest composer known to him, acknowledging his profound talent and compositional knowledge.
Step-by-step explanation:
The person who told Wolfgang Amadeus Mozart’s father that Mozart was the greatest composer he knew in person or by name was none other than Joseph Haydn. Haydn, a prominent composer himself and a pivotal figure in the development of classical music, recognized Mozart's exceptional talent. During one of their interactions, Haydn said to Leopold Mozart, "Before God and as an honest man, I tell you that your son is the greatest composer known to me either in person or by name. He has taste and what is more, the most profound knowledge of composition." This moment is often recounted in the history of music to illustrate the immense respect and recognition that Mozart earned from his peers.