I'll narrow it down a bit for sanity's sake... No expansions, no bundles, and no handheld games.
With that said, I only see these games having a shot at the top:
1. GTA IV
2. Mario Kart
4. GT5 (unlikely that it will launch WW in 2008 IMO)
Killzone, MGS4, and some of the other games I see listed here don't have a snowball's chance in Hell of topping those games. The only FPS that could even think of hanging with those four games is Halo and even that hanging with GTA and GT is unlikely. It's hard to accurately judge the Wii's install base so MK and SSBB just turn mediocre sales (4-5m) just as easily as they could go ballistic (8m+).
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