Best of all, so far.

The Microsoft OWA for iPhone would not even connect to my company server. Other apps didnt like any extra screens the server might throw.

This app works OK with my companys server, but has the following shortcomings:
1) Will not send to a .net email address (likely a bug).
2) I will pay to get rid of ads! At times, they occlude buttons.
3) Lacks a quick return to the Inbox folder.
4) Lacks favorites setup for folders.

AZDanTheMan about OWA Webmail