awesome topic.
1. Ma'a Nonu. Nz rugby has a simple plan through the ages and we always execute it better with a dominate 2nd 5. Ma'a is the player that offers the go forward in midfield that the 2nd phase feeds off, forget his long pass, forget his effective kicking game, forget his defensive management of the area 2 passes from the ruck, Ma'a is the man the opposition have to defend against because he will run straight and hard into midfield, lay the ball back offering quick play the ball (because our forwards also know he is going to run straight and hard) so all the other backs in black now have space, simple but effective and he can do it all day, SBW does not offer this (see Wales game).
2. Conrad Smith. the only current inside back (unless Carter can get his form back) that is not a confidence player (meaning does not lose it when things are going wrong), he manages those around him when times are tough, he runs the defence, and is arguably still our only true centre.
3. Richie. Leadership, ref management, 80 min effort, turn overs, his game
4. Ben Smith. like Conrad, not a confidence player, offers cover during a game from 2nd 5 out, (could play 1st if asked), safe and always thinking, guaranteed metres in attack.
5. Read. Offers a point of difference skill wise that forces teams to target him in defence which opens up opportunities.
in my opinion all other players are replaceable, perhaps they are better than others but replaceable.