Friml wrote his most famous operettas in the 1920s. In 1924, he wrote Rose-Marie. This operetta, on which Friml collaborated with lyricists Oscar Hammerstein II and Otto Harbach and co-composer Herbert Stothart, was a hit worldwide, and a few of the songs from it also became hits including "The Mounties" and "Indian Love Call". The use of murder as part of the plot was ground-breaking among operettas and musical theatre pieces at the time.  After Rose-Marie's success came two other hit operettas, The Vagabond King in 1925, with lyrics by Brian Hooker and William H. Post, and The Three Musketeers in 1928, with lyrics by P. G. Wodehouse and Clifford Grey, based on Alexandre Dumas's famous swashbuckling novel. In addition, Friml contributed to the Ziegfeld Follies of 1921 and 1923.  Friml wrote music for many films during the 1930s, often songs adapted from previous work. The Vagabond King, Rose-Marie and The Firefly were all made into films and included at least some of Friml's music. Oddly enough, his operetta version of The Three Musketeers was never filmed, despite the fact that the novel itself has been filmed many times. In 1930, he wrote a new operetta score for film, The Lottery Bride. Like his contemporary, Ivor Novello, Friml was sometimes ridiculed for the sentimental and insubstantial nature of his compositions and was often called trite. Friml was also criticized for the old-fashioned, Old World sentiments found in his works. Friml's last stage musical was Music Hath Charms in 1934. During the 1930s, Friml's music fell out of fashion on Broadway and in Hollywood.

Answer this question "What was his greatest piece he released?" by extracting the answer from the text above.
In 1924, he wrote Rose-Marie.