The top of my list is Dragon Valley. (I know, I can hear someone in the back shouting "But they are still open!") I'm talking the old school Boys village site in West Aberthaw. By todays standards, it is a fairly tiny site. But back around 2000-2002 when you saw photos of players walking through what looked like streets playing airsoft, when all you had was a woodland site that was predominantly paintball, so all of the wooden structures where constantly covered in orange and green goop. It seemed like airsoft heaven. At the time, on the "Ol' blue" forums it was the site that everyone talked about. I made two trips down there, once for a normal walk on game (A good few of the #airsoft mIRC crew where present) and once for a weekender event which was dominated by the WASP team (It was the event where for some reason Silvafox wanted his boot prints on the ceiling of the safe zone above the stair well, so some guys lifted him up and stuck his feet to the ceiling.) I was between 17 and 18 when we went down and it felt like the wild West. Super laid back, the general consensus was "don't be a dick" and "take your hits and everything will be fine" and the atmosphere was amazing. Part of me thinks it's just nostalgia that makes it seem so amazing, but it was a great little site and it stood out in the community. On a side note. The first time I was there, I knocked myself out in the empty swimming pool, the second time I broke my kneecap. Bloody good times!
A site that gave me a similar vibe to DV was SWAT Saighton camp. "Ol Satan" was a site that a lot of us sort of took for granted. We didn't really know what we had until it was gone. Massive site, with huge indoor areas, lots of buildings and roads. Plenty of weird spots to hide and a great community. The filmsim games they used to run where loads of fun. The Terminator game stands out, I was on the Skynet team and relived the police station scene from the first movie by just casually AF walking into the resistance CP with a SAW and wiping the whole place clean.
If time travel holidays ever become a thing, or if somebody makes total immersion video games that are indistinguishable from reality. I'm re living the super happy fun times at those two sites.
other sites that are gone but not forgotten:
Ambush Strines (where I lost my virginity, not "that" virginity... The one with the balls being fired at you)
SWAT Area 51
SWAT Fortress
Wentworth CQB
Labyrinth CQB (I know F&O have it now, but it's just not the same)
The Stan was always a good laugh