I was tempted to say Let It Be too, because not only is it about the Beatles breaking up, but it has some really good stuff from Harrison and McCartney.
However, I think the Golden Slumbers Suite REALLY shows off everyone at their best. Paul's voice on Golden Slumbers is beyond beautiful, and the instrumentation behind really captures the heart.
The Beatles had some REALLY strong harmonies going on during Carry That Weight, and I loved the trumpet sound from the orchestra playing.
The End had Ringo's only drum solo during the Beatles-days, and had that very groovy guitar battle between Paul, George, and John, which showed how these three could really rock on guitar. Let's also not forget, "And in the end, the love you take is equal to the love you make." Sounds like the Beatles philosophy... at least I would hope that's their philosophy
. And in those great chords at the end, and it is (to me anyway) their real swan song, not Let It Be.
And believe me, it pains me to say that because LIB is my favorite Beatles songs and got me into listening to them more.