It is really rare for a theatrical presence to maintain himself or herself decades or centuries after their death. Prior to the invention of motion pictures it was impossible (except for the late 19th Century crowd, starting with Edwin Booth, Sir Henry Irving, and Ellen Terry - they had some phonograph recordings) for actors to preserve their personas. Photo stills helped but did not leave much. Occasionally one would "accomplish" something for good or ill that people would recall (i.e., John Wilkes Booth), but that was a rarity. Movies changed that by allowing the audience to stretch from the contemporaries to the future ones. There are actual movies of Flo Ziegfeld at work on Broadway. Unfortunately he did not look like Bill Powell (Flo was somewhat fat), but those surviving newsreels showed his energy with his casts and productions. Powell got that side of him right.
THE GREAT ZIEGFELD traces the showman from his start as the publicity manager of Eugene Sandow (Nat Pendleton) from the Chicago World's Fair of 1893 to his rise as a major Broadway impresario from 1900 until 1930. Only the Great Depression destroyed Ziegfeld's career (he lost most of his fortune in the crash). Significantly his fall is showed by the lights of his four currently big shows (including SHOWBOAT, RIO RITA, and THE THREE MUSKETEERS) all fading out one after another. It not only destroyed his career, but it helped kill the poor fellow (he died in 1932). But by that point his historical reputation was established by his series of spectacular Broadway "Follies" shows with the likes of Ed Wynn, Fanny Brice, W.C.Fields, Will Rogers, Bert Williams, Walter Catlett, Leon Erroll as the comic leads, and music by such composers as Jerome Kern, Victor Herbert, Irving Berlin, and even the Gershwins. But better was Ziegfeld's use of women - his "Glorifying the American Girl" reviews did precisely that, affecting taste in theater spectacles and taste in what beauty in women should be.
He had his rivals as pointed out in other reviews here. Among them were the Shuberts, possibly the most successful in the long run (but only because there were several of them, and they did not go bankrupt). Earl Carroll did produce his "Vanities", and George White did do the "Scandals", but in retrospect (for all the talent they brought to their shows) these were pale imitations. In Carroll's case he skirted the level of decency by suggesting his chorus girls were naked in some of the sketches. Ziegfeld was no saint but he knew what was acceptable behavior in theatrical production on stage.
This film had a good cast. Besides Powell, the women in his life (Anna Held, his chorus girl girlfriend, and Billie Burke) are played well by Louise Rainer, Virginia Bruce, and Powell's movie "wife" Myrna Loy. Rainer (as the betrayed Anna) got her first "Oscar" for this role, most likely for the emotional "telephone" scene. Her part is actually a substantial one, but due to the size of this film it has always been seen as a short supporting bit. This is a trifle unfair. Virginia Bruce brings a nice calculating eye to her performance, undermining the Ziegfeld - Held relationship, but slowly losing the showman due to her increasing alcoholism. Loy was really lucky. Once she got the role she contacted the living and active Billie Burke and had discussions about how to accurately portray the second Mrs. Ziegfeld. She wisely does not try to imitate Billie's familiar syrupy upper register voice.
Frank Morgan appears as a friendly rival or Ziegfeld's, both in business and in the boudoir. One likes him as one usually likes Morgan's comic characters. My favorite performance is that of Fanny Brice playing herself. The film was made the year after the death of Will Rogers so that he was not around (he possibly could have been borrowed). Fields was out - Ziegfeld had had plenty of trouble with that curmudgeon on Broadway, and MGM probably knew that Fields would have demanded equal billing to Powell. Wynn was mostly still on Broadway. One does wish Catlett and Erroll might have been used. But we have Fanny in her glory, culminating in her singing her most famous number - "My Man". That alone is worth watching the film for. As for the showcase production number on the spiraling tower, it is quite impressive (I doubt if Ziegfeld could have put it on the stage of his New Amsterdam Theater), but I agree that the dubbing of Dennis Morgan's voice by Allan Jones is totally inexplicable to this day.