Great question, but for me it depends on what mood I'm in.
A Day in the Life has been often singled out as being top-of-the-heap regarding individual songs, but sometimes it's the little quirky ones that get my attention.
For example, on the (BluRay version of) Why Don't We Do It in the Road, Ringo's drum intro is incredible to me: what trickery is going on? (Listen to it again loud; the take was recorded in cavernous Studio 3, set up for an orchestra the next day, which gives it the echo.)
According to Jeff Lynn, Electric Light Orchestra was formed because he was so impressed with one Beatles song: I Am the Walrus.
"I buried Paul" anyone?

