Forgive me, but I am using your post for part of my answer, because you bring a lot of questions to the table.
Best offense how so?
For 1 season, or are we talking a core group over a few seasons?
That matters because if we are talking 1 season I have different answers than if we are talking a core group. The 1970's team was together......first, before I was born, and second, was before free agency. It makes a difference.
I am going to answer this question and put my own clause that it has to be for one season.
Top Offense:
2009 Ariizona Cardinals - That offense was practically unstoppable. Warner, in my mind was at his best in his career. I do not remember a team that could go out and just embarrass teams like that offense did.
2015 Arizona Cardinals - (With a nod to the 2014 squad). I will ALWAYS think to myself. What if Tyrann Mathieu doesn't get hurt, what if Dwaste wasn't a waste. What if this team peaked later on in the season. The 2009 offense was plain deadly from anywhere. This offense was the most explosive offense I have every witnessed live.
I never saw the 1970's Cardinals live, since I was not alive. I would have to reverse engineer the team based on stats, but at a glance, give me the year Otis Anderson had the most rushing yards. The Cardinals could always pass, but I could not imagine Anderson at his peak, the dude was HUGE for a running back.
Best QB: Kurt Warner, the 1-armed, glove version, guy was locked in and close to a madman every game.
Best WR: Fitzgerald, 2008. If you are a fan that has been alive during his career and missed the playoff games he has been in, then I feel sorry for you. No one is even close. Boldin in 2005 was special, that is a season where as a Cardinals fan you watched one of the best seasons any player can have, and the rest of the NFL world was oblivious to it. I will tell you someone that is in the top 5, maybe top 3, David Boston. If we are talking 1 season, David Boston is on the list, and John Brown in 2015 is in the conversation.
Best OL: N/A
Best RB: David Johnson 2nd year was pretty special