Honestly, if you're happy using the native workout app, I'd stick with that.
Generally, the other apps fall into one of two categories. Either they are specific to another service (which generally pre-dates the apple watch) such as Strava or Nike Run Club, where you can compete against friends. Or two, they offer an improvement on the workouts app. As an example, the native workouts app can give you a vibration at the mile markers, but the Nike Run Club app, which I use can be set up to give you verbal notifications, as frequently as every .25 mile, with your time, distance, and average pace.
Each app also seems to suffer from issues every now & then. The Nike App, after working great for me for almost a year, started to have issues with a recent update. The Strava app, while really good, has a very sensitive auto-pause feature that will pause (for me) every time I glance at the screen. I've run with the Native Workouts app before, and like it quite a bit. If I weren't invested in the Nike Run Club system, I'd probably just use that.
The one big advantage to the third party apps is that most of them allow you to view your run data from a web browser on any device, while the native workouts app only allows you to view your data in the Activity app of your iPhone.