S.T.A.L.K.E.R.: Shadow of Chernobyl
And if you add good modifications it gets even better.
It has an unequaled depressive atmosphere, the out-of-the-box textures work is already awesome, the music is chilling and very well fitting, the A.I is quite good despite a few bugs and limitations (though as of version 1.0005 vanilla the A.I is better that's without a doubt), there's not too much action to fit the context, but not a lack of it to get boring, it has that perfect balance of lonely "exploration" and scripted events. The A-Life system, especially when well modified turns the somewhat static Zone of the vanilla version into a living breathing unpredictable and very dangerous place.
I just love that game, and its prequel,
S.T.A.L.K.E.R.: Clear Sky looks even better, which surprisingly enough is scheduled for a Q1 2008 release. And despite the 3+ years delay of Shadow of Chernobyl I do believe they'll be able to meet the deadline this time around, because they're just upgrading their engine, they're not making a completely new one, which would have been a whole other story. And look at how they upgraded the graphical branch of their engine (amongst many other things, including a DirectX 10 renderer):
http://www.stalker-game.com/en..._frame.php?id=10&res=0 (modeling example)
http://www.oblivion-lost.de/in...p?zone=design2&lang=en (modeling example + screen-shots)