The Beatles prioritised the mono mixes right up until Let it Be/ Abbey Road. For purists they are the way to go as they reflect the bands's vision of the music more accurately than the stereos, as these were often done at a later date without input from the musicians. There are also significant differences in the mixes, for instance many people consider the mono Sgt. Pepper to be the definitive version.
So what do you need to buy to get all the original mono albums in mono and the original stereo albums in stereo?