For me it's Blackstone for the reasons already mentioned, but also good for those of us using public transport being only 20 minutes walk from the station.
Another site I'd recommend is South Herts Airsoft near Kings Langley. While facilities are a bit sparse the atmosphere is always friendly, the owners do listen to feedback (both positive and negative) and are regularly working on the site to keep things fresh. To me the only negative is that it's difficult to get to and I'd definitely go there more often if it wasn't.
This. There's been quite a few times where I've had great days and supposedly bad sites and there's some sites with excellent reps which I wouldn't return to if I was paid