I miss two sites.
The first is UCAP Sandpit; as others have mentioned, this site had everything: CQB, wide open spaces, areas with loads of cover, the pipeyard, vehicles etc etc. A massive site with great gameplay.
The other is Doomsday in Kent; it was a small, scruffy and very laid back site that was superbly run by people who understood the absurdity of grown men running around shooting each other with toy guns. Although rather small, it always felt bigger (ooer missus!) due to the layout and the varied terrain. They also did the best veggie food in airsoft.