This is too hard to isolate one album each. You see, I think Lennon's "best work" is of several types, and is scattered through the albums, almost to the end. Macca is an early low player, and a late great player. Anyroad, here's my take:
John Lennon: A Hard Day's Night is his early massive showcase. In studio years, John's contributions to Rubber Soul, Revolver, Pepper and MMT are to be revered.
Paul McCartney: Until their studio years, Paul had no showcase album. John was either there too, or dominating. In their studio years, Paul's contributions to Rubber Soul, Revolver, Pepper and MMT are to be revered also.(That's why these albums are so brilliant.). Paul's showcase album is really Abbey Road, isn't it? All his qualities (plus & minus) are there.
George Harrison: I'll go with the White Album for George. He doesn't (sadly) have a showcase like Lennon & Macca do. However, George's guitar work is one of the pleasures of listening to the Fabs; in a sense all their album's are Harrison showcases.
Ringo Starr: He's in a similar position to George. Without the mortar of his playing, the Fabs would be not so fab! The albums where I enjoy his work most areprobably ... er, here we go again ... Rubber Soul, Revolver, Pepper and MMT!!