Where can the highest air density be found on Earth? I would like to see if there is any possibility of testing, validating and actually demonstrating a potential design for drag-mediated rapid braking of an ultralight plane - possibly a solar plane. In reality, a drag coefficient is not really a constant - it varies with both density and speed.  

Tests at low density can be done using altitude, but for the highest density points, I am not sure even what densities are ever available anywhere, and how to go about finding these locations.

At first I thought - Death Valley or the Dead Sea area at night, but they can still be fairly warm. 

Then I thought the south pole is really cold but then I realized the altitude is quite high.

Then I thought flying over some arctic or antarctic area near sea level would be the highest density, but water can warm the air a bit.

Is there a known place where one could fly through the highest density air on Earth? Is there a world's record? I suppose one could at least analyze historical airport data, but you'd need temperature, pressure and humidity and synthesize a density.

Assume the craft is actually flying and has a pilot on board - no environmental chambers, wind tunnels or mine shafts.
Any place in the arctic near sea level on a cold day would be much lower than the dead sea on a cool day.

Density Altitude Calculator