Define this however you like. The most fun to play. The most complicated. The most melodic. I want to know your favorite Beatles basslines, and what's so great about them. For me, it's Glass Onion. It's a mix of counter-melody, which also has a steady chug that pushes the song forward. I also love the string attack, it's so brutal that you get the extra cool percussion effect of the strings clacking against the frets. I remember hearing that as a young teenager, and it grabbed me by the throat and demanded my sense of awe. 30 years later, I'm still examining this one bassline and trying to understand all of its beauty, complexity, and simplicity. To me, this bassline makes the song.