I have tried most of these apps, and each have flaws, but agree that Outlook does some of the best job. The main issue for all of these is how they handle Yahoo emails (don't ask why I still keep a Yahoo address). Connection to Yahoo servers is excruciatingly slow, and a lot of spam goes through. Therefore, my main criteria for apps are how fast they connect, how they show updated badges in real time, and how they display content. Another key factor is quickly understanding who is cc-ed or not, without this taking half of the screen.
AirMail has issues with formatting some emails with pictures, where the text is unreadably small if there is a picture included. It also tends to be a little slow to update. If many people are cc-ed, it takes too much space on the screen.
Edison has the best real time badge update, but it completely fails with Yahoo accounts and jumps all around, going to the top of the page while loading content and crashing a lot. I reported the issues many times but the app is unusable in my hands.
I do not care for the Spark interface not taking advantage of the entire screen.
Outlook is amazing at parsing important mail from other listings, and is overall the most solid. I have my work account there.
I continue to use iOS Mail for privacy for my main personal mail accounts, and because it is stable and displays content properly. However, it is slow, does not do push for Gmail, will not let me see real-time badges, and has very little customization options.
Overall winner, Outlook, then ioS Mail. Others don't compete.