I picked Yellow Submarine. It's not that it's necessarily a bad album, but it's a mish-mash of previously released stuff with a few new tunes mixed in -- a semi-compilation.
Let it Be wasn't their best work but it's hard to dislike an album which contains Get Back, Don't Let Me Down, I've Got a Feeling, Let it Be, etc.
Beatles for Sale may be their most underrated; it was the first album which showed the Beatles were changing musically, becoming more introspective. It was the first stepping stone to Rubber Soul.
And it surprises me that Help! doesn't get the praise that the albums recorded afterwards got -- it seems to be dismissed as a film soundtrack; but there are some awesome songs! Besides the title track, you get Ticket to Ride, You've Got to Hide Your Love Away, You're Gonna Lose That Girl, It's Only Love (I love this one).