My favorite is The Beatles Revolver.
Revolver is, IMO, the best transitional album - the songs are all approachable, yet remain experimental. The execution is polished.
While many will say that Sgt Pepper is the Beatles’ best album, over the years I found myself leaning more toward Revolver. Pepper is a great concept album, but there are only a few memorable songs. Most people have heard the majority of Revolver at some point in their life.
So if I were to pick one album that represented the Beatles at their height as a pop music band, it would be Revolver.