For me, it has to be during sunrise. I once took a flight from Luton to Edinburgh that took off just before sunrise, and the cabin was bathed in a fantastic orange light for the first 20 minutes of the trip. I'll not forget that experience in a hurry. And no, the orange effect was nothing to do with EasyJet's cabin colours
A close second would be night flights over large towns and cities. On a Virgin A340 out of Dulles heading to LHR, the whole of Wash. DC and the Maryland / Viriginia suburbs was lit with a splendid orange sodium glow which went on for as far as the eye could see (perfect weather that night). It was breathtaking. I'm glad that I don't do that trip very often, because it makes me appreciate it a whole lot more when I do see sights like that.
I guess it's obvious; I'm just a sucker for views from thousands of feet up
~NJTurnpike (who's considering a name change to 'WindowSeatFan' ! )