Wrath of the Lich King Still on Top
Most people don't follow it but every week the NPD group, widely known marketing and industry gurus, produces a list of the top selling PC games. How much stock you put it in the list is up to you, since it doesn't cover digital distribution like Steam or Direct2Drive but retail sales. Either way, it seems Blizzards World of Warcraft Expanison Wrath of the Lich King is still king of the list.
Imagine if they were in Steam...