I voted for Rubber Soul, partly because of so many superb songs on it but also, admittedly, because of memories associated with it (tough to dissociate those from 'critical thinking') but perhaps first and foremost: it marks such a sophisticated advance for the boys - one that showed them their future path.
Abbey Road is a close second - the lyrics are so creative, so confident, exquisite really. Mean Mr. Mustard is still out there somewhere waiting, I know. Not to mention Maxwell's Silver Hammer (Paul said they came to use the term for anything bad that unexpectedly happens). They really were at the height of their powers. Several years ago, I heard an interesting story on NPR about how grammar school teachers use the songs in their classes. Children like them for their 'jauntiness' and playfulness and the fact that the Beatles "sing high" like they do. What a wonderful legacy.