I really like Outlook, Spark, Boxer and Airmail. I'm also looking forward to Polymail to become available on iOS soon.

I'm now using Outlook on my iPhone and iPad but it does lack a few features which e.g. Spark or Boxer have like but those apps are missing other features :( - attach multiple files from cloud storage location in one go (only Spark can do this) - search in e-mails older then 3 months (Spark, Boxer, Airmail all can) - snooze to desktop (like Mailbox has) - save e-mail as PDF (like Spark, Airmail can) - short/long swipe gestures (like e.g. Airmail) - task/action list of e-mails (like Airmail) - TouchID / password (like Boxer has)

My comparison: http://forums.macrumors.com/threads/e-mail-apps-that-support-idevice-cloud-attachments.1912468/