Best starting 5 is tough, partly because the team would need balance to be good, i.e. offense/defense, inside/outside, and balanced scoring among the players.
'86 C's and the 72-win Bulls proved to be the best of their time, even if each player wouldn't make a best-5 on their own merits (e.g. Danny Ainge, any Bull not name Jordan or Pippen).
I gotta say, if they choose to play the regular season like it means something, Rondo, Ray, Pierce, KG, and J.O'Neal have the potential to come together like those squads did, and I think may have similar postseason success.