Google's In Box or the GMail app. If I really have to use something else, I'll use K9 Mail. But I don't ever have to use anything else.
I automatically forward all of my email to my Gmail account and have my gmail set to reply using the address the mail was sent to, so I only need the Gmail app, plus I have filters in Gmail to automatically apply a label with the name of the account the mail was sent to, so it's always organized. About once a week I go into the other accounts natively and clean them up (mostly just delete all of the messages.)