Best "Old" site would be between two early Hot Shots sites - The first I played was in a former hospital down near Alton in Hampshire. They then ran a site using lots of old accommodation blocks at Greenham Common. Epic gun-fights at both playing alongside some real paintball names from the time.
Best current site imo has to be CPPS tournament fields at Penkridge - The fields, the playing surfaces, the facilities, the investment, the attention to detail and the pure passion to get it right by Ainsley/Dylan and their team of minions is simply astounding. Weekends spent at the site have a real "event" feel about them just because you have so many ballers in one place at one time. The only downside if there is one is the fact that so many people want to play the event (myself included) that you get little to no opportunity to spectate any games for yourself