If the aim is to get out without too many additional loses and to limit Iranian influence Sadr may be the best bet. We have made the army and the police the main repository of firepower in Iraq and they are heavily infiltrated. Given that the top dog is likely to be Shia then they need to have enough control to be able to be independent. If they are too weak they will turn to Iran for support and in so doing will have to accept some loss of independence. I doubt – regardless of ideology – that any of the main players would want to be Iranian puppets if they could be leaders of their own state (albeit with Iran as their closest ally). If not Sadr who has the clout without going cap in hand to Iran? I know we are getting down to picking from a list of very unpalatable choices but if we instead anoint someone we prefer we increase the risk of a disastrous civil war whose humanitarian consequences the world will not forgive us for.