Looking through the Fall release dates...I think it is a given that the best selling game for their respective month (Aug-Dec) will be on the X360.

Aug - Madden 2010   8/14/09, this is an annual thing so 100% sure

Sep - Halo 3 ODST   9/22/09, it is Halo (Marketing, Marketing, Marketing) 100% sure

Oct - It is either going to be Forza 3  10/27/09, Splinter Cell Conviction 10/20/09, or Halo 3 ODST. These are the only major games coming out in Oct (in the case of ODST, the momentum could carry over from Sep like what Halo 3 did.).  95% sure

Nov - Modern Warfare 2  11/10/09, I think everyone in the world knows MW2 will be the biggest selling game this year. 1000% sure

        note: you also have 3 others great games coming out on the X360 in Assasin's Creed 2, Bioshock 2, and L4D2. All 3 of them will be in the TOP 10 in Nov. 99% sure

Dec - Again December will be dominated by a COD/MW game just like the last 2 years.

Read Title...US only

That's entirely true. Despite the Wii's success, the Xbox 360 has an awesome presence in the US market.

I don't know... we'll have to wait to see what Wii Sports Resort does.

As far as MW2 being the best seller of the year... I don't see it beating Wii Fit, even. Wii Fit has already sold over 3 million in the first half of 2009. It may outsell Wii Fit in November and December, but not for the whole year.

What about the wii vitality sensor

trust me thats gonna appeal to LOADS of wii owners, it can be huge if marketed right

Wii Fit. Wii Sports Resort.

You also have to take into account the 90% probable Pricecut/Slim of PS3... with a probable bundle of a big game to go with it.

But, MW2 will DEFINATELY be up there.

for me ODST is the only interesting but whatever.

My guess:

first -second option

Aug: Madden(360) - WSR
Sept: Halo ODST - WSR
Oct: WSR- Halo
Nov: Modern Warfare 2 (360) - Wii fit
Dec: Wii fit- WSR

And don`t forget New Super Mario BRos

While probably true for the one month periods, I have no doubt that each of those games will get bested by Wii Sports Resort, New Super Mario Bros, Mario & Sonic Winter Olympics, and Wii Fit Plus with lifetime sales.

Are you forgetting about Wii Fit Plus? That game is out this fall (Sept/Oct), I think it's going to sell really well, like 1 million or more first week.

Wii Sports Resort will also be big and NSMB will not be big enough IMO.