STALKER: Shadow of Chernobyl
Wow, that was quite a trip. It weighed in at almost 40 hours in the end, and some of the later levels were very punishing, what with having to manage ammo, health, radiation, carry weight, all while fighting off dozens of enemies of various kinds, as well as the environment.
I had to back out of one level halfway through to go back to base and stock up on more medkits. Luckily the enemies generally stay dead in STALKER and I was able to start back where I left off. The most fun in a way, is the start of the game, where you are quite powerless.
It has its flaws, but if only every game had its ambition. Highly recommended.