BioShock
It is not just another game with awesome graphics thrown in, it’s much more than that, it’s an experience - it makes you feel, disturbed mostly, and makes you think about what happened to Rapture: Politics, science, ethics and morality. But it doesn’t end there, as the player also has a moral choice to make: To harvest ADAM¹ from the little sisters - creepy little girls that recycle ADAM from the dead - and kill them in the process, or save them by killing the parasite inside, gaining much less ADAM making you less powerful. Tough choice.
There are few games with as much substance as BioShock. If you haven’t played it already and you have an XBox or a PS3, you really should play it.
¹ ADAM are stem cells, produced by an underwater slug, that allows genetic mutations.
There is also a cool PDF with the game’s art and concepts. Get it here (bottom of the page).