Terminal 4, Madrid Aeropuerto
I'd heard that the architecture of this airport was well-done, but wow. Certainly at the top of the list of those I've been to so far. The rolling brown and yellow roofing just goes on and on, and the whole thing is a big open area. It's gorgeous.
I also had to spend an unintentional night in Madrid, thanks to weather in London. Comparing the hotel Iberia put me up in to the conference hotel I'm in now, I can honestly say I'd rather not have done that.