Basically, a point guard distributes the ball and makes his team mates better by giving them good looks at the basket or an easy basket, this is what Nash does for the Suns, but you also have to consider he is playing with all-star caliber team mates, which makes his job easier. In my opinion, the only problem with Nash is that he is only effective in the type of game the Suns play, when they are forced to play half court offense they suffer, the only half court play that I see them execute often is the pick and roll.
Here is my list of point guards who really helps their respective teams (Not in any particular order):
Chris Paul - Hornets
Steve Nash - Suns
Chauncey Billups - Pistons (Mainly for his knack to hit big shots)
Deron Williams - Utah
Jason Kidd - Nets
Stephon Marburry - Knicks (Joking, though he is a great one on one type of player)
Can't think of anyone else at the moment, its worth mentioning that if Chauncey was there for making big shots then Gilbert Arenas should be there too, well I guess, he has been playing well and their playoff bound. I'll put in Kirk Hinrich just because I am a Bulls fan
, cheers