That really depends on what you prefer and need. The UI can very a bit and also some have features that others don't have. See also
the comparison sheet(look at the images especially) I recently updated in this other forum topic from a while ago.
Overall
Airmail, Spark and
Email by Easylido are great options feature wise.
There are many more e-mail apps out there though as you can also see in the comparison sheet.
Ok I just tried Email by Easilydo. Here are a few observations:
1. Suffers from the same issue as Airmail it seems: it doesn't show a pin icon for all the emails with attachments. Spark does, though. I don't know what seems to be the problem.
2. "Undo Archive" button seems to work after archiving an email -- but today it seems to work in Airmail as well (it didn't work last time I tried).
3. Text editing options when composing an email are nonexistent. Should add more.
4. Adding pic attachments to an email seems to be possible only 1 by 1??!? Plus no info whatsoever about the size of the pics you are about to send. Not good...
5. Attachments shown on top of the mail when you open it is a good thing.
6. Smooth interface.
7. Still needs work, so it won't become my primary mail client for now (if at all).