No more anecdotal advice: Final verdict is rear center seat. Simply put, it is the furthest seat from most of the angles your car is likely to be hit at in a crash. Impact is the single biggest killer/injuring factor in a car crash (yes I know, genius). So standoff distanct from the impacting object is the single most important factor outside of a secure restraint itself. Can't sit them in the middle for whatever reason? It doesn't matter which side they're on at that point. Just don't seat them in the front.
From the ppl who keep stats on what kills people (the CDC):
Place children in the middle of the back seat when possible, because it is the safest spot in the vehicle. They were quoting Committee on Injury, Violence, and Poison Prevention. Child passenger safety. Pediatrics. 2011;127(4):788-93. www.cdc.gov/motorvehiclesafety/child_passenger_safety/cps-factsheet.html
Though there was no statistically significant difference in risk injury between children seated behind either the driver or passenger seat, the researchers found that children in the rear center position had a 43% lower injury risk than children seated in either side position. What could explain this significant finding? It goes without saying that vehicle crashes usually impact the front, rear or side of a vehicle, so children who are seated on either side and are involved in a side-impact crash are more at risk of injury than those seated away from one of the sides (i.e. the center). In a side-impact collision, the researchers found that rear center-seated children were at a 54% decreased risk of injury. But, even in collisions where the impact was other than one of the sides, rear center-seated children were still at a statistically significant lower risk of injury. http://www.pedsforparents.com/articles/4019.shtml
In general, the single safest place in the car is the center rear seat, because it is farthest from the outside of the vehicle. In any given crash, however, a different seat may be the safest, such as a left outboard seat in a right side impact. http://www.carseat.org/Technical/tech_update.htm#seatpos