These two would be the obvious choices, being the stories where the characters even actually do get some characterizatization.
The big problem with the film adaptations would be laying out the setting of the First Age properly to audiences who would not be familiar with the story. It would need a lot of worldbuilding to get it right.
The Silmarillion is my favourite book and I would love to see it done right. There is hope, even if it is slim.