I would never show it in that order to someone who truly didn't know anything at all about SW since it would ruin what is probably the greatest 'surprise' in the history of motion pictures in ESB.
Though for anyone other than a small child or who has been in cryogenic suspension or Outer Mongolia for the last 40 years that has probably been Spoilered by now. [I feel sorry for kids today who didn't get to experience the Big Secret Revealed in theatres...it was genuinely SHOCKING in those days when there was no online for spoilering movies and studios could actually keep secrets from audiences until they actually saw the film...everyone in the theatre really DIDNT KNOW and it hit us like a BOMB it was just AWESOME lol]
For anyone 'unSpoilered' or small kids I'd show it in order of I, II, Clone Wars, Rebels, IV, V, III, VI imo.