This is my favorite genre, so I've been attempting to find the best games also (but I don't own every system...yet). Here's a few PC games:
Oblivion - one of the best games out there that you can alter to your liking with mods.
The Witcher - compelling storyline, new additions to the genre, really good.
Mass Effect - haven't tried it yet myself, but I heard it was great.
Fallout 3 - definitely a game you should try out.
