Here are the fitness apps that I'm using:
Under Armour Record
I'm a runner. When I'm running outside, I use MapMyRun and strap my iPhone to my arm. If it's raining, I put my iPhone in a ziplock bag before strapping it on my arm. Now, I always run with my iPhone.
You'll see me wearing a Jawbone UP24, so my data syncs with my iPhone and I use the UP mobile app to track my activity progress. Don't have an UP band? There is an UP app that works with the built-in motion sensor in newer iPhones.
I use this to track my indoor and outdoor activity. My Polar V800 syncs with my Polar Flow app.
I don't wear a Fitbit anymore, but it was one of my first activity trackers, so I continue to stay active within the Fitbit ecosystem of users (I joined the Fitbit community in 2011). I use the Fitbit app and the MobileTrack option that works with the built-in motion sensor in newer iPhones.
I mainly use RunKeeper to log my running miles. It's redundant. It's an app that's very similar to MapMyRun.
I'm still experimenting with Apple Health. It's there on my iPhone, so I'm going to try it out and feed it with data from several sources.