If you are looking for On-Demand games, I would suggest getting either Batman: Arkham City or Far cry 3
There's a few other ones that are good too.
Just picked up Arkham Asylum and Arkham City Game of the Year Editions on the PC for $5 with the latest Humble Bundle. Makes the Marketplace's pricing look high.
If Warner are willing to work with Humble to do deals like this then I wonder why they, and other publishers, aren't willing to do the same with Microsoft.