My favorite for my gmail is Blue Mail, for the following reasons:
I loathe conversation mode in the Android gmail app, it's very hard to distinguish each of the replies to a thread. Gmail allows you to disable conversation mode for other mail accounts, but not for gmail accounts.
While Outlook lets you disable conversation mode (even on gmail accounts), it always seems to have problems refreshing the view - messages disappear after you have read them, and come back in to view some time later. Additionally there is no way (that I was able to find) to set your sender name in the Outlook app (for gmail accounts), so all your emails come from "whatever@gmail.com" instead of "Joe Blow".
Blue Mail lets you disable conversation mode and set your sender name.