Fable!!!
One of the biggest problems with PC RPGs is that they are almost all massively multiplayer with some heafty subscription fees. Fable has to be the second best single player PC RPG that I've ever played (sorry, I've still got to hold out on Elder Scrolls: Morrowind).
The story is really good, with some pretty good plot turns.
The only thing I would really wish for to be changed is the limited weapon/magic selection. Also, it was so great, I wish it had been longer.