I was a great fan of Kevin Johnson, but he's getting overrated in this thread. I think Suns fans are partial to him because he played almost his entire career in Phoenix and led the franchise out of one of its darkest periods. But putting him ahead of Nash really doesn't make sense. Nash was a better shooter and a better leader, and he dominated the (offensive) game in a way that Johnson didn't.
Johnson had a couple of things he was terrific at -- the little pull-up jumper from 15-18 feet, and beating almost anyone off the dribble -- but his range was limited and he had difficulty finishing in traffic. I remember too many times how the Suns would need a basket late in the game, they'd set up a play for Johnson to get to the rim, and then he'd get there and miss, while everyone cried for a foul. Was he fouled? Sure, probably, but part of being a great player is either getting calls in your favor or somehow getting the job done anyway.
Also, if you're going to evaluate someone's contributions to the franchise, you have to take health into account. Johnson played at least 70 games in a season only once after turning 25, and his career ended after just 735 games.
Here's a hypothetical: If you swap Johnson's and Nash's eras, so that Nash plays with Barkley, Chambers, Majerle, etc. and Johnson plays with Stoudemire, Marion, etc., what happens to each team? I think the Barkley team improves with Nash instead of Johnson, whereas the Stoudemire team looks about the same to me.