For group discussions the first requirement is that it's one that other members of the group use. Then it's a matter of whether you need it to be cross-platform, whether it needs to integrate with other apps or services, whether you need to have an archive that's accessible to new members. There's no single "best app", it depends on what you are using it for.
I don't know what "best for mobiles" actually means. And "best for learning", what is it you want to learn, and what device do you have (e.g. a note-taking app like LectureNotes is of limited use if your device doesn't have stylus support). In any category the "best" app will depend on your particular requirements.
As for driving, the best app is the airplane mode switch so you aren't distracted.