Have tried Outlook, Gmail and Inbox. They are all good but just like most apple products, the mail app is simple and easy. For me, simplicity normal trumps all.
The native email app is pretty good. The way the iPhone handles email is the biggest issue I have. I've heard rumors that this will be addressed in the next major update.
I use the default app. I don't ever send emails, just receive, so the stock email app works perfectly for me. I like the way it looks, how simple it is, and the functionality it has.
Stock is pretty good but for some time now I have used outlook as it has integrated calendar in it which is pretty handy. After a month I noticed that I didn't use it all that much so I'm giving the stock one another try now. Imo the less you need 3dt party apps in your phone, the better. If Apple keyboard gets swipe type someday I don't need SwiftKey anymore.
I use the stock mail app as well. I have not really tried any others on iOS. The stock app does what I need and as iOS evolves it seems to be getting better with attachments which was my biggest issue with it. Looking forward to iOS 9.