JPJ- Man, Zep II was a huge album for bass. Ramble On and The Lemon Song both kick ass, but I always liked his playing best on What Is and What Should Never Be, just so much groove, quintessential bass playing.
Entwistle- The Live at Leeds album really showcases the full power of his bass playing. I especially like Young Man Blues, he and Keith Moon just so completely hold it down.
Jack Casady- Again, I find his live work is where he really shines. His tone and his playing are just so sublime and heavy. I guess if I had to pick one tune as quintessential Casady bass, I'd go with The Other Side of This Life. It's on Bless Its Pointed Little Head.
Phil Lesh- Once again, it's all about his live playing, but that should be a given considering the band he played in. It's hard to really pick a song that showcases his playing particularly as it varied so much from performance to performance. I guess a Dark Star from 1969 to 1974 or a Playing in the Band from 1972 to 1974 could be a good example of the breadth and depth of his playing when the Dead really went out there. I've heard everything from gentle melodies to heavy grooves to hard, proto-punk chords to wall-shaking feedback over the course of one of these jams. For his quicker playing, I'd say an Eyes of the World from '74 is a good place to look.