For me, it was the mid to late 2000s.
I was fortunate enough to experience the days of Lightfighter Airsoft in Yorkshire; primarily the fantastic Clay Wheels site in Sheffield.
It was, and still is to this day, the best site I've been to.
A large, built-up industrial site ran by competent staff who actually cared about Airsoft.
They'd run decent, well-thought out days that could range from normal open days to dedicated theme days, and in-between.
Numbers were good, but not over-the-top. You never felt like sardines.
Contrast it to today, and unfortunately, very little remains in the majority of sites I go to now.
Many tend to be your standard paintball style woodland sites with open, bare ground and wooden or tyre barricades, or they're wooden CQB speedsoft fests.
And player numbers can often be way too high for the size of the site; higher running costs obviously mean that owners will be looking to do everything they can to pay the bills.
I still enjoy it somewhat, but the shine it used to have for me is long gone unfortunately.