I have several great third party camera apps but hardly use them because you can’t replace the built in camera button on the lock screen, control center, and the Messages app with these third party things. I hope iOS 12 allows this for Camera along with Mail, Calendar, etc.
Initially that was a huge pet peeve of mine. Over the last few iPhones however (6S Plus, 7 Plus, X), unless I'm experimenting with RAW (or want extreme HDR), frankly I prefer the output of Apple's camera app.