Paul McCartney--everything on Sgt. Pepper, but the bass on With a Little Help from My Friends--well, it's full of melody, invention, and rhythm.
Paul McCartney--kill me if you want, but I think his bass line on Silly Love Songs is just perfection. Rhythm, melody, memorable.
John Deacon--My Best Friend: His run downs add a classical element to the song and still manages to rock.
Dee Murray--He would be considered great even without these, but I love his work on Elton's Funeral / Love Lies Bleeding.
Chris Squire--Everything really. I've always been in awe of his playing. One great moment: The bass lines on Parallels from Going for the One.
John Entwistle--Again, everything. I love his bass on Won't Get Fooled Again.
I could list thousands, as I imagine all of you could do as well.