That's a really tough one.
While their earlier albums during the height of Beatlemania, from Please Please Me to Help!, were really good, it was Rubber Soul where they REALLY started to become the greatest band of all time in my eyes. That one is my personal favorite. But to call it the best one is another matter entirely. The White Album is right up there, as is Abbey Road and Sgt. Pepper.
... Man that's hard to choose. But I think I'll give the slight edge to Abbey Road.
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