This past year I've also watched Rome, I Claudius, and The Borgias.
Downton Abbey is my favourite of all of these because of its attention to detail and slow pace. I enjoyed it almost as Poldark, which is perhaps my favourite Masterpiece Theater series of all time.
Its the attention to detail that makes the difference. HBO had it in Rome, whereas Pillars of the Earth seemed rushed. I haven't yet made up my mind about The Borgias and Game of Thrones, both of which I'm enjoying. It's hard to pronounce an evaluation of a series before it is finished.
I think Fellows may be challenged to bring Downton to a graceful close, but will say no more at the moment to avoid spoilers.