Based on the IGN Wii LTDs, public NPD data, and selective leaked data (for the reason that leaked data is included, I will round to the nearest 100k so as to disguise the nature and number of data that is not allowed to be used), the INDIVIDUAL SKU top 10 would include.
Halo 3 (~4.1 million + December)
Wii Play (~3.0 million + December)
Call of Duty 4 360 (~1.6m + December)
Mario Party 8 (~1.4 million + December)
Madden 360 (~1.1 million + December)
Super Mario Galaxy (~1.1 million + December)
Guitar Hero 2 360 (~1.0 million + December, data might be off by a bit)
Assassin's Creed 360 (~0.9 million + December)
Guitar Hero 3 360 (~0.9 million + December--but this might be thrown off by the fact that individual SKUs might separate GH3 and GH3 w/guitar)
Super Paper Mario (~0.8 million + December)
That's what I have. Now, if you include the DS/PSP, Diddy Kong Racing and Pokemon DP should both be there. I'm also ignoring the possibility that anything not on the top 10 going into December would overtake any of these.
Resistance is around 600k year to date (missing November AFAIK) and I don't have numbers for MotorStorm past month one. With an aggressive December Assassin's Creed and COD4 could potentially also be up in the contender range.