I have some spare money and I've decided I want to get into Pc gaming. Which game should I buy?

I am an extreme xbox addict so I have all the games that are out on both pc and xbox so I'm kind of looking for an exclusive title because chances are I'll already have it if its dual platforms. Im interested in either getting counterstrike source, or the witcher 2 so far, but I'm open to suggestions. I don't particularly like the look of terraria, and I've already dabbled in minecraft, but any other suggestions I am open to. Preferably modern titles but I'm open to ideas.