Not counting the obvious Wii Sports, Wii Play, and Wii Fit, it'll probably be Galaxy, Brawl, or Mario Kart. All three have the potential to be the best selling of the lot, but there isn't enough to say what order they'll be in. So far Brawl looks like it'll outsell Galaxy, but I have absolutely no idea about Mario Kart.

Mario Party 8 is selling insanely well right now, but I have a feeling sales will drop like a rock after Mario Party 9 arrives. There will definitely be 5M+ selling third party games, but I doubt they'll be quite in the same league as the best first party games.

I am going to guess a close call between Mario Kart and Smash Brothers. I am going to go with Smash because I like it more, but I can just as easily see it losing to Mario Kart.

Are you certain it will be included with Mario Kart wii?

I think Galaxy will come in behind Brawl and Kart thanks to online play. After beating Galaxy, there's not a lot of reasons to replay it, and it's an easy game to loan to friends. But with Kart and Brawl, being able to play against friends is a good reason to hold onto that disc - and your friends who want to play will have to get their own, too.

yep I think either mario kart wii and smash bros brawl will be best selling wii game
one of them.

Why exclude Wii Fit? That's like excluding Rock Band or Guitar Hero because they come with accessories. And YES it is just like that.

Besides Wii Play, Wii Sports and Wii Fit. I'd say Brawl, then Galaxy, then Mario Kart. Maybe. I don't really know. They will each sell well over 10 million.

You can exclude Wii Sports for being a console pack-in, but you can't exclude Wii Play simply for being inexpensive, and you certainly can't exclude Wii Fit for being MORE expensive than a normal game due to the "pack-in" balance board.

Wii Music we don't know enough about yet. We can't say yet if it will be another Wii Sports, Play or Fit. If could be a 20M seller, or a 3M seller.

Don't forget Animal Crossing Wii. AC for DS is going to reach 10M. I think a lot of that will carry over to Wii. The Wii edition may sell worse in Japan, but it may also sell better in America.

I'll say...

Wii Play- 30M +
Wii Fit- 20M
Mario Galaxy- 14M
Mario Kart- 11M
Animal Crossing- 10M
Smash Brawl- 9M

I won't speculate about unannounced games.

 You shall have your comeuppance, good sir. You hear me? Comeuppance!

Mario Kart Wii: 18 million
Smash Bros Brawl: 17.5 million
Mario Galaxy: 16 million