Some of my favorites...
SEA - Absolutely gorgeous with Puget Sound, especially when the approach takes you within what seems like spitting distance of Mt. Rainier.
SYD - Depending on the approach great opportunity to see the Harbor Bridge and Opera House.... not to mention depending on the winds it almost feels like you're going to land on water.
HKG - Another great one with city views albeit I prefer the old airport better. :-)
LHR (depending on approach) - I've had the occasional approach which provides a spectacular view of the city proper including Tower Bridge, Big Ben, and London Eye.
DCA - Always cool to see the monuments from above!