I don't know about the "best" app, but I just installed ARGUS which is a health/fitness app that tracks things like how far you have walked, calories, weight etc...
What was interesting is that the M7 processor must be constantly tracking and logging information even when you aren't using it with a specific app. For example, I just installed ARGUS today and it filled in all the steps I have taken each day since I got my iPhone 5s back on Sept 25th.