I would have to say Wii Fit is going to win, out of the games I know of right now. Then again, I'm sort of including Wii Fit + in Wii Fit.
I think you're underestimating New Super Mario Bros. Wii though. 8-10 million? Really? It will sell at least 3 million first month. Since it's releasing around christmas, and has very little competition, it will probably do more. I simply cannot see it stopping at under 15 million in the long run. It even has a chance of 20m+.
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