Tough question. I would have said it's a toss up for me between We Can Work it Out and Day in the Life - and those were the two mentioned above!
If I have to make a choice, I would likely say A Day in the Life. Musically, it's just brilliant. The ultimate build up of sound... it's one of the most intense climaxes I've ever heard. I love that it's equally a Lennon/McCartney song, they both had bits here and there to contribute to make this incredible masterpiece. Ooh every time I hear it, it still blows me away!