1) Dwarf Fortress (no one else thinks this is awesome?!)
2) Planescape: Torment
3) Arcanum
4) Ultima VII
5) Alpha Centauri (+ Alien Crossfire)
I'm playing Fallout 2 right now (and it looks good so far), so the list may change!
I mostly love darker atmospheres, solid stories, well-crafted worlds, interesting characters, flexibility in playstyles and heavy doses of choice and consequence. And stupid dwarves.
Where is Witcher :'(