I'm going with GT5. The multi-generational flagship game for a system should outsell a game like MW2.
I think an exclusive game should be the top selling game for a system. I wouldn't want MW2 topping Halo 3 (which it might very well do unfortunately, htere's always hope for Halo: Reach though). PS3's top selling exclusive is only at #4 (MGS4). God of War isn't going to come close so it's up to GT5 to show the exclusives proud.
Gaming consoles have flagship games, games that everyone associates with the console brand. Those games are meant to be among the highest sellers. Though I do notice PS2's number 1 game was a multiplat, with a GT game coming in 2nd place. That's the only time among the current console makers a non-exclusive title has been the #1 game.
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