What I dislike about the standard mail apps is that if I eg delete a mail, the focus jumps to the next (unread) mail and marks it read, regardless of me having read it or not, or wanting to read it right now or not.
For me, this is so incredibly annoying and I can't really cope with this 'feature' in my daily workflow. It sometimes even makes me overlook important emails.
I'm therefore using Spark. It does not mark emails read autonomously, has a couple of nice features, runs on Mac and iOS, is reliable, and support so far has been fast and good.
@Apple: I sent feedback on this mark emails read issue a few times already, but so far you have not considered it worthwhile doing something about it. Shame.