I can't believe so many of you are saying Wicked, which would be comparatively EASY to turn into a film (albeit an expensive one)!
Impossible:
A Chorus Line (Ooops! They did it anyway.)
The Drowsy Chaperone
Pacific Overtures
The Mystery of Edwin Drood
Any of the revue shows: Ain't Misbehavin', Black & Blue, Sugar Babies, Jerome Robbin's B'way, Fosse, Movin' Out, etc. Unless you just film the stage show... which really isn't a film so much as a "documentary" of what was done on stage.
Extremely Challenging:
Sunday in the Park With George (people walking around in his painting could be very cool or very disappointing, depending on the art direction and SFX--which would be too costly for this kind of film). And the music and book wouldn't easily adapt, either.
Follies - would have to be overhauled completely to work on film
Not Easy but worth the challenge:
Ragtime
Merrily We Roll Along
Nine
Grand Hotel
Into the Woods
A Little Night Music - I wish they'd try again, but they won't
Once On This Island
More or Less Ready to Go with some adaptive changes:
Wicked
Jersey Boys
Big River
The Secret Garden
Kiss of the Spider Woman
Don't bother:
Titanic
Sunset Blvd.
Les Mis
"Jaws is the Citizen Kane of movies."
blocked: logan2, Diamonds3, Hamilton22