"The best musical adaptations I find are always based on decent or meh films that have either been forgotten or had stronger potential and the musical brings their strengths and emotional resonance to the surface better than the movie could."
I sort of agree - my favorite screen to stage adaptations (some are based on films I think are also great, but the adaptations radically diverge from the source):
Grand Hotel
Nine
A Little Night Music (this one diverges least from the source)
A Gentleman's Guide to Love and Murder (although the writers disingenuously claim that it's based on the novel)
Grey Gardens
Little Shop of Horrors
A Catered Affair
Least favorite? Many, including Ghost, Rocky, Chitty Chitty Bang Bang, The Little Mermaid, Tarzan, High Fidelity, Leap of Faith, Shrek, The Addams Family, Priscilla, 9 to 5, Sunset Boulevard.