Most if not all of the 3D Mario games have been a GotY contender. What sets Galaxy apart from the rest of them for me is that it's the only one that has actal world building, character development, and story telling. It's not the same old "Bowser kidnaps Peach. Mario go save." Even though that does happen here, there's so much more to it than that. There is so much atmosphere and scope to Galaxy that is absent in all the others. I'm trying to help and save Rosalina and the Lumas, bot because I have to in order to beat the game, but because I WANT to out of the bond and connection I develpoed with them as a player through Rosalina's Storybook and interacting with them. Having the greatest spundtrack I've ever heard in a Mario game also helps tremendously in that regard.
Galaxy is just a massive and thrilling adventure in scope and fun from beginning to end, because it's not the usual Mario experience, there is so much more to it than that.
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