Odd is against Madrid for 2 back-to-back European Olympics or 3 if you count the 2014 Winter O in Sochi, Russia.
Rio is a dark horse. IOC really wants Olympic to be held in S. America for the first time but safety and high crime rates are a major issue. Rio got the lowest score of all 4 finalists, as a matter fact Rio placed fifth but IOC over-rided the fourth placed Doha, this may piss off the Middle-Eastern IOC members.
I think the choice is going to be between Tokyo & Chicago. One wild card Chicago has is Obama as he is wildly popular overseas. If he wins the presidential race, Chicago's chance will increase tremendously. So I'd vote for Chicago if Obama wins, Tokyo if not.