Which was the first Beatles’ song which showed Paul as a virtuoso bassist?
This one for me: http://www.youtube.com/watch?v=MPjDMZiuhbQ
And your ol’ pal Quas’ is primarily a drummer, so I am used to working with bassists.
You may wanna wear your headphones!!!
We’ll work on Mr. Harrison next. 
Then Mr. Lennon…
…And Mr. Starkey.
Then all of them together.
And yes, I know Mr. McCartney was multi-instrumental, okay, y’all?
Thanks
Quasi
Bassist checking in. Great question/topic. I agree that the “Paperback Writer/Rain” single is an example of virtuoso playing and put McCartney among the upper echelon of rock bassists during that era. His note choices, feel, phrasing, instrument switch, and recording technique were superb. However, I think he began demonstrating his virtuosity on Rubber Soul. Sure, he plays a lot of 1-3-5 on it (as he had on previous albums), but on RS he really found his groove/voice and began to get more creative with his lines. He takes chord tones and adds rhythmic variation & syncopation and the tunes really come alive. Tracks like “You Won’t See Me,” “Nowhere Man,” and “Drive My Car” are a few that come to mind. In my opinion, they provide a glimpse of the brilliance Sir Paul would be creating in the next phase of the Beatle’s output.
I think his playing stands out more on RS than on Revolver. Save for the well-known line on “Taxman,” a lot of Revolver’s lines are pretty by-the-book. Don’t get me wrong, the dude served the songs. Restraint is a sign of maturity and (not to sound too cliche) “less is more.”
From Sgt. Pepper on, McCartney created some of the best and influential lines in bass history. If “Something” or “Hey Bulldog” (to name just a couple) are not examples of virtuoso [electric] bass playing, I don’t know what is.
I don’t know if I answered the question :), but I’m definitely curious what other musicians think.