S.T.A.L.K.E.R. - Shadow of Chernobyl

Fallout 2

System Shock 2

Bioshock

Metro 2033

Resident Evil 4

Deus Ex

all of the above fit the requirements rather nicely.

Planescape: Torment

Final Fantasy X

Spec Ops: The Line

Fallout 1, 2, 3, New Vegas.

10.Bioshock

9.Okami

8.Mass Effect 2

7.Metro 2033

6.System Shock 2

5.Half-Life 2

4.Dragon Age: Origins

3.Planescape: Torment

2.Fallout 2

1.Fallout 3

for me it's definitely the Fallout overworld, i also liked the universe the Culture novels are set in but i preferred Fallouts for its darker tones.

Planescape's a black isle game, look it up