I think that the S.T.A.L.K.E.R. games are probably among my all-time favorite games. They rank up there with Doom to me, because the horror moments are intense as fuck and scare the shit out of you. The rest of the game is me running around a bandit camp with night-vision at midnight, sniping them and running from tree to tree between each shot. They start to panic and fire in random directions as I switch locations and hunt them all down, until there are only a couple left and I shoot out the lights, pull out my shotgun and move in for the final kills.
What a great series, and it's under-rated