Coolest part of the fridge
Most refrigerators have the cold air outlet on the top, that way the cold air sinks down naturally to the bottom. Air from the bottom is then circulated through the heat exchanger to make it colder and then goes back in the air outlet at the top. So the very coldest place is right in front of the cold air inlet in the top. That's generally below freezing though, so you may get things colder than you want. 

Other than that the coldest place tends to be the bottom shelf. You could also cool it down in the freezer for awhile and then transfer it to the refrigerator.