Final answer:
The most important musical partnership of Billie Holiday was with saxophonist Lester Young, whose seamless interplay and close relationship with Holiday produced some of jazz's most expressive recordings.
Step-by-step explanation:
Billie Holiday's most important musical partnership was with Lester Young. While Louis Armstrong was indeed a central figure in jazz and an incredible improviser whose influence resonates to this day, and Bessie Smith was known as 'The Empress of the Blues', it was Lester Young, a tenor saxophonist who had a particularly close musical relationship with Billie Holiday. They had a profound mutual respect and understanding that translated into their music, leading to some of the most memorable and emotionally expressive jazz recordings. This partnership is often celebrated for its seamless interplay and heartfelt communication, encapsulating the true essence of jazz.