Modern Warfare 2 has little chance of becoming the best selling title.
World at War sold 6.5M in the first year. Release date was 10th November 08.
Modern Warfare 1 opened with 5.7M. Release date was November 5th 07.
Modern Warfare 2 releases November 10th.
Now, how much will Modern Warfare 2 need to sell to take the first place? (Save Wii Sports, of course). Personally, I'd say 10M is lowballing Wii Fit a bit.
How about second? I doubt that too. Wii Sports Resort is currently entering the holiday season at the same pace as Wii Fit and MKWii did last year. They ended up selling 5.3M and 5.5M from here on and in. I don't think WSR will manage 10M for the year, (that would be 5.3M more), but it should come rather close.
3rd? I think Mario Kart Wii will steal that. Its sales seem linked to the Wii, so if the pricecut boosts the Wii, it could make 8M for the year (that's 3.5M in the holiday season). 7-8M is likely.
It should make 4th (assuming you also exclude Wii Play) , though. DQDS is looking at ~ 4.5M for the year, as is NSMB (DS). Resident Evil 5 might outedge it, though, with ~ 5M.
Other possible competitors are NSMB Wii (I consider that unlikely). Mario & Sonic @ the Winter Olympics (should sell ~ 5M, if it does as well as the Summer Olympics) and Wii Fit + (that's ridiculously unlikely)
