I've been an avid amateur gamer since Windows 95, which is to say, quite a long time, and the S.T.A.L.K.E.R. universe is one of my most favorite places to be. My second favorite would probably be Fallout 3 and Fallout New Vegas as they both also have quite a number of very good addons available. A very close third would be the 'Metro 2033' series.