Main stay is the DJI stock app as it's pretty much rock solid. But Ken's Ultimate Flight is a very close second. Lets face it, Ultimate Flight has so much more to offer then the stock app and it's pretty slick. Though, I'm still having a few issues but this may because of user error. Ken's support of the app is unbeatable.
I also bought the Vision Pilot app and don't think much of it. It should be considered in alpha stage but the developer is charging top dollar for it.