I'm a self confessed third party productivity app addict, so after reading Frederico Viticci's review (see link below) and having used Airmail on OS X in the past I had to give it a try.
I'm using on my iPhone and iPad, and love the extended third party app integrations (calendars, storage, task managers etc), it is highly customisable and can do almost anything.
If eye candy more your game, I'd look at Spark from Readle. If you are in a professional environment with Exchange, try outlook. To be honest, the stock standard Mail.App option is still highly viable and does a good job, and continues to get better. Good to have choices, however right now I actually have all installed on my iPad (yep I'm crazy).
Anyhow back to Airmail, for me it does everything that I need and has proven to be rock solid and highly reliable. If it wasn't for the odd choices of design in Spark on the iPad, I wouldn't have bothered to check out Airmail. Also if you are a keyboard user, as I am now with the Smart Keyboard Cover, Airmail allows you to customise what the arrow, return & delete keys do, meaning a email workflow where you don't need to touch the screen to open, read, action or archive/delete emails.
Link to review -
https://www.macstories.net/reviews/with-version-1-1-and-an-ipad-app-im-switching-to-airmail/