I notice in the EMS chart that only half of the IR range will travel through the earth's atmosphere. So in the range of frequencies that penetrate the atmosphere what frequency travels farthest?
I suppose what you want is the graph at http://en.wikipedia.org/wiki/Radio_window, provided you are talking about vertical transmission from space to the earth's surface. Results would presumably be different for horizontal transmission because the pressure and chemical composition of the atmosphere would be much closer to constant, but I can't immediately source those transmission coefficients.
As you see, radio waves transmit best, 10 micron comes through pretty well, etc.