I think I would look more at the emp effects. Take out the power grid and then step back and let nature take its course. Very little residual radio activity, very little damage to infrastructure and max damage to target. Time the detonation say for around 9:00 am so that the population is out and most active. Can you imagine how many people would be in elevators at that time. How many cars and trucks would be out moving around. Max disruption at the least I think.
Now that being said, ya'll have a good day.