"I Want To Hold Your Hand" is solidly the song that, in my view, best represents the "invasion" period, the early music of the Fab Four. Today's song, even with it topping the charts in mid-1964, provides a few clues (the bridge, the fade ending, the one-note opening) that winds of change were coming, not only to Beatle music, but to American society as well.
Then, I believe "Hey Jude" is the definitive song of the later Beatle years. That's just personal opinion, and there are many songs others may point to as well. That's the great thing about doing a countdown like this. I get the chance to offer you my musical tapestry of my first 50 years, and you can chime in as well! :)
Here's John, Paul, George, and Ringo with their fifth contribution to the Top 500 with, yet again, another number one hit. :)