It depends on what you're looking for in a running app and the data you're looking for.
If you're looking for online stats, Endomondo works well. If you're looking for data only on your phone, Track Runner, in my opinion, is the best stand-alone run tracker on Windows Phone. All of the data from a run that a runner would look for. In addition, it tracks your usage of different gear and the weather conditions for each run (weather is automatic).
Personally, I now use the Band and then have it linked up to Strava.