I'd put another vote for Stalker, best fun I've had in ages. Although some of the bugs can be frustrating, I never experienced anything that bad. The games has a very cool atmosphere, and playing in the underground labs late at night for the first time was a chilling experience. The best part is how the AI works. Stalker uses something called A-life, I think, and it governs the actions of all of the NPCs. If you raid a bandit camp, eventually, the bandits will come back and repopulate. However, there are other factions as well, and they can come and take over just as easily.
My first play through of the game, I encountered something so awesome that I just had to keep on playing. In the beginning, you start at a rookie camp, and down the road is a military base. One of my first missions was to kill a bunch of mutated boars near the rookie base. As I was traveling to the boars, I saw two or three soldiers marching up the road, patrolling. I hid behind a bush, because I thought they would shoot me. It turns out, the boars ventured to near the soldiers, and the soldiers opened fire. My PDA chimed, they had just completed my task for me. Being the greedy bastard I am, I wanted some of those assault rifles the soldiers had. I tried to kill them with my measly pistol, and lured them into the rookie camp. Suddenly, an alarm went off in the military base, spouting Russian. The camp is overrun with soldiers, and I, and the rest of my rookie friends, are bravely defending our homes. After a long firefight, the soldiers retreat, and I help myself to some much needed bandages and ammo, lots of ammo.
That fight was very, very challenging, yet so much fun. Later, as I was sneaking into a bandit camp, I saw a circle of bandits sitting next to a fire, listening to a friend play the guitar, I cried when I was done killin.