Of course this Age of Aquarius/Let the Sun Shine In is overplayed, but I love it anyway. I forgot the name of the fellow who plays the smokin' bass (especially on part 2), but I think he's in the session musician Hall of Fame and might have been part of the Wrecking Crew.
Per the wikipedia page, these were the bassists who played in the Wrecking Crew at various times:
bass: Carol Kaye, Joe Osborn, Max Bennett, Chuck Berghofer, Ray Pohlman, Larry Knechtel, Lyle Ritz, Red Callender, Jimmy Bond, Bill Pitman
I recognize only two of those, Carol Kaye and Larry Knechtel (who played on the Partridge Family records, iirc), but I imagine it could have been any of these.