While there are plenty of movie musicals that could successfully be reimagined and remade, there are some where a straight redo would be as pointless as Gus Van Sant's Psycho.
The Wizard of Oz: The 1939 musical isn't perfectly made but its blend of elements has a magic no straight remake could hope to recapture. A musical - or indeed non-musical - based more closely on L.Frank Baum's book could work well.
Cabaret: The 1972 movie reimagined the Kander and Ebb musical in a way that worked brilliantly for the screen - all the songs took place in the cabaret or otherwise live and commented on the action; the character songs were eliminated except as background music. A remake would have to incorporate most or all of the original score and establish an identity of its own.
Gigi: Lerner and Loewe's 1958 original movie musical is a slightly risque (even disturbing if you think too hard) classic and it's hard to imagine anyone bettering this cast, especially Maurice Chevalier, who managed to make Thank Heaven for Little Girls sound charming rather than creepy.
Meet Me in St Louis: This 1944 look at the life of an American family has a beautiful balance of light and dark, sweetness and poignancy, and an ideal cast. Don't touch it.
Seven Brides for Seven Brothers: With its fantastic dancing and enjoyable - and original - song score, this musical overcame its stagey setting to become a classic.
Singin' in the Rain: Often seen on lists of all-time great movies, this 1952 classic set at the time when sound film replaced silents should not be tampered with, but simply enjoyed and appreciated.
The Band Wagon: Another MGM classic and an unlikely candidate for a remake - the story is flimsy - but there are some excellent song and dance numbers and t's hard to imagine the performers, especially Fred Astaire, could be bettered.
The Sound of Music: The 1965 box office smash became the default version of the Rodgers and Hammerstein show. Julie Andrews is indelible as Maria and while the film is schmaltzy and omits the more cynical songs from Max and the Baroness, and adds and changes a few other songs and bits and pieces, it's too well established.