S.T.A.L.K.E.R.: Clear Sky. By far the weakest link in the series, it's even buggier than the first game, and not nearly as atmospheric due to the over-abundance of NPCs in the game world. It has some good ideas and the first stage (the swamps) is awesome, and so is the new Red Forest, but the rest of the levels are either rather boring re-hashes of areas from the first game, or very linear and poorly paced battlegrounds that feel unfinished (Limansk and the old hospital).