Why would the Suns trade their second best player when they have the best record in the West?

You mean the best player on their team. Marion is the only one on the team that had great offense and was above average on defense. If you're referring to Nash as their best player hahahaha that's laughable. Nash doesn't know the meaning of the word defense or taking care of the ball especially against the better point guards in the league. The Suns would be out in the first round regardless if they had Marion. It's simple defense wins championships. The Suns don't play any D. Therefor will never win a championship.