There's also this nice other option of comparing the base SKU to the base SKU.
NPD tracks the games separated, so if the question is what game has the highest monthly total in NPD, it makes sense to report it as they track it. If NPD hadn't changed how they report numbers to the public, I seriously doubt we'd be having this conversation, anyway. :lol
Edit:
If you want a combined SKU Top 5, it'd be ...
1. 360 CoD: Black Ops - 4.9m (Nov-2010)
2. 360 CoD: Modern Warfare 2 - 4.2m (Nov-2009)
3. 360 Halo: Reach - 3.3m (Sep-2009)
3. 360 Halo 3 - 3.3m (Sep-2007)
3. XBX Halo 2 - 3.3m (Nov-2004)