You need an option "ALL" LOL. Seriously, they were great throughout all periods of their music. I simply cannot pick just one. I love the Sgt. Pepper's, however, but the White Album was phenomenal as well (save Revolution No. 9). And yet, there is Abbey Road, which is another gem. All are from different periods and exceptionally good. Also, I am not sure if you can cite Yellow Submarine as a true Album since it was composed mainly of already existing music of theirs, except for a couple songs like "Hey Bulldog" and "All Together Now".