What's the best photo editing app that makes them look natural?

What's the best app for iPhone to make pictures look natural but look really pretty & clear also what's an app that collages pictures without having any outline at all

I don't have an iPhone but I found this app called the Pixlr Express and I use it alot I love it. It can make your photos vibrant. And if you got bag under your eyes I was able to fix that a little bit. It is a great app I'm not sure if it is the best, but it's the best I've come across.