Bioshock 2, but admittedly it was on sale for just $20. Only later did I realize that my computer didn't have a chance in hell of actually running it. Fortunately, I gave it to a friend with a computer good enough to run it, but that was overshadowed by the abominable griefing scheme that is Games for Windows Live (We did get it to finally work, but it was quite an unpleasant ordeal).