I think you do need three categories.
I've always thought of everything up to and including Help as being early Beatles as they are all sort sort of similar and kind of like the same band improving all the time.
My favourite album from this period is Hard Day's Night - worthy of note as the only Beatles album where every song is by Lennon/McCartney. There's so many great tunes on it, a personal favourite being You Can't Do That which I think Lennon was actually pretty critical of later on.
Also, of the 13 songs on Hard Day's Night, I'm reasonably sure 10 were by John Lennon - a pretty amazing statistic when you think of McCartney's contributions to the last few Beatle albums.
For me, Rubber Soul and Revolver don't sit too comfortably in either early or late Beatles not only because of when they were made but the sound on these albums.
George Harrison himself says on Anthology he thought they could have been a double album they sound so similar. I also think it's these albums that establish them as geniuses - there were hints of it on Help with I've Just Seen a Face and Ticket to Ride but on Rubber Soul there's a hatful of classic Beatle songs - Drive My Car, Norwegian Wood, Nowhere Man, Michelle, In My Life - you get the idea.
Revolver speaks for itself - just over half an hour of some of the greatest three minute pop songs ever written.
However, Revolver is definitely a 'normal' album in that it pretty much followed the template of previous Beatles albums in simply presenting a bunch of the best of their new songs.
As everyone knows, Sgt Peppers Lonely Hearts Club Band pretty much wrote the book on how bands make records today.
It's ground-breaking approach definitely represented a departure for the Beatles and as such I would consider it the beginning of their 'late' period.
The rest (White Album, Let it Be, Abbey Road, Yellow Submarine (if you can call that a Beatles album) and the cobbled together - but nonetheless excellent - Magical Mystery Tour) is history.
So, in summary, er, I don't know which is my favourite period. They all rock.