Entourage X, for a few reasons.
One, I have been using an MS email client since OE4.5, and it is just easir to import from one MS client to another (I have had brief stints with Eudora and Mail 10.1, neither worked out for me).
Also, I liek that everything is there in the one app, not jumping around to see calanders/to dos/memos addresses like you do with the Apple apps.
I have been looking at changine over to Mail.app 10.2 though, as I haven't tried it yet, and using it (and iSync to sync everything at once) would make things a whole lot easier.
--PB