My list goes as follows:
Planescape: Torment
Demon's Souls
Baldur's Gate series, if I had to choose one, the second one.
Dragon Age: Origins
KotOR series, as with BG, if I have to choose one, it has to be the second one, with Restored Content mod, that is.
As a side note, the top 2 are also two parts of my top three games of all time.
EDIT: Hmm... I completely forgot Morrowind. I don't know which one of the five above to take out, so I'll just have to say that Morrowind is my hidden 6th. It has to be in there somewhere.