I think An Béal Bocht by Brian O'Nolan would make a good film, although I don't know how it would play outside of Ireland.

I always thought that the Tales of Bauchelain and Korbal Broach series by Erikson could make good movies, entertaining and complete all by their own

Not really sure, when I read a book I don't really think about whether it would adapt well.

On the stage. Filmed plays rarely work well without significant adaptation. A good play can take meaning and structure from it's medium, change the medium and those elements no longer work. Also there is a very different emotional response I experience when watching a play than to that when I watch a film, in theatre the experience is much more intimate, more alive and almost real. I have laughed and cried far more often in a theatre than I ever have in a cinema.

By the way I'm talking about straight adaptation from play to film. For example last year's version of Hamlet by the BBC. While the performances were great and it was interesting to see the whole piece never really worked on film. Compare this to the film10 Things I Hate About You. It was an adaptation of a Shakespeare play which I felt really worked, because they actually adapted the play as they might adapt a book (in other words making appropriate changes) rather than just taking the script straight from the play.