I actually had to answer this question on the first day of my Set Design class this quarter, and to come up with an answer I just sort of opened Winamp, looked at my five-star songs and tried to determine which one I had loved the most and for the longest and most consistent amount of time.
The answer I came up with was "You Never Give Me Your Money" by The Beatles
and I actually stand by it as my all-time favorite song. There are only two songs that could work as the Last Song Ever for The Beatles; this or the actual song they finished with, "The End" ("Her Majesty" is discontinuity to me, only symbolic in the fact that it displays the fact that for all the amazing things The Beatles did and what a great rock band they were, they also made a lot of dumb songs too). Everything about this song is fantastic, and it really paints this portrait of a relationship falling apart (as The Beatles had), the subject of the song growing up (as The Beatles had), and that bittersweet feeling that all of this reality and this relationship finally ending is looming in the near-future, but that's not now.
It's about the realization that this was the last album The Beatles would ever record, and how whilst it's a sad thing, it's not worth dwelling on the bad and instead enjoying it for what it is and ending on the right note.
Basically, the three pillars of The Beatles' music, to me, are "A Day In The Life," "Strawberry Fields Forever," and this. "Strawberry Fields" is the first Beatles song I ever fell in love with, the crescendo to the big final note in "A Day In The Life" will always give me goosebumps, but "You Never Give Me Your Money" is a perfect song. The band shines here (Abbey Road
as a whole should be trotted out as a "shut the fuck up, dude" comeback whenever anyone tries to assert that Ringo Starr is not a talented drummer), the lyrics are infectious, and the song shifts through three different moods and overall sounds effortlessly, showing off some of the different styles the group had absolutely mastered by the time they called it a day. And now once I get home from failing to work on my Play Analysis paper I'm going to have to play it on Rock Band again.( Coming up, on the song meme! )