I currently just use the Apple's Activity app which works well enough. I've tired Nike's app and a few other things, but none of them really stuck. I was actually considering switching to FitBit's app because if have a lot of friends with FitBits. That didn't last long though because FitBit and Apple don't get along so there is no direct integration from FitBit into HealthKit and vice versa without a 3rd party app.