I think that "Best All Whites" is, at it's heart, two questions - who are NZ's best players, and who played the best for the All Whites.....and they aren't the same thing.
NZ's best players would be definitely Rufer at the top, then Nelsen, then the likes of (in no particular order) Thomas, Reid, Killen, Wood, Elliott, Vicelich....those guys who had good careers in good leagues.
Those who played best for the AWs; Nelsen for pretty much running the show in 2010, then Sumner......then the likes of (in no particular order) Coveny, Smeltz, B. Turner, Almond, Vicelich, Elliott, Wood etc - guys who played a lot for us over a long period of time and achieved something.
Marrying those two ideas, Ryan Nelsen would be my best All White. Simon Elliott, Ivan, Woodsie all up there, Thomas will be one day, There's no way I'd consider Wynton Rufer a great All White, but he was our best player.