That's a pretty general and subjective idea. The best for them as human beings–health, finances, and entertainment? The best for them in the classroom? To connect with other educators? To stay on top of emerging tools and trends in education? To use with students?
For iOS, Android, or Windows Phone? Can we assume there's WiFi access? Is data use an issue? What about data privacy? And what do we mean by "free" Truly free? Are in-app purchases available? Necessary? Is it a free version that has hideous banners everywhere?
To say that there is a lot to consider is an understatement.
That said, we've taken a wide-lens view of the modern teacher and taken a stab at what might be considered 50 of the best really, truly actually free apps available for iOS. They may have some paid options, but they're each entirely useful without spending a penny.
While we do look at music, health tracking apps, and other teachers-as-a-human-being apps, we focus mostly on the kinds of digital content that will help you teach more effectively to a wider range of students in more compelling and dynamic ways than ever before. All free.