Galaxy 1 and 2 complement each other well enough that I'd consider them perfect together.
Galaxy 1 was fucking amazing with the tone set in the game, setting a clear space motif, but also giving the worlds atmosphere (sometimes bodaciously) to keep it from being "planetoids in the inky blackness of space". The sub-plot was great, and even what they did with the plot was an improvement (the character growth of the luma with Mario) over previous games. However, because of the high prevalence of more space-related levels, the colors used in many galaxies were very dominated by browns, blacks, and greys, which is odd for the vibrant Mario games, where even monochrome castles usually have lava or fire. I also took a bit of issue with the fact that the alien spaceships appeared at the beginning of the game, but largely dropped off with no explanation. You even find some crashed in a late level. They didn't seem like something Bowser would develop when he and his minions are still using airships. It just feels like something they could have expanded upon. There were also some control issues, and it sometimes felt like the game wasn't quite precise enough, and when coupled with having to wrap your brain around spheroids, this could lead to some frustration. The game was overall fairly easy though (Luigi's Purple Coins can fuck off, especially when you do it as Luigi), even when you died from control/camera/whoops issues, so you never really had to worry about running out of lives (though, not like losing all of them really did much IIRC).
Galaxy 2 improved the gameplay so much, and fixed the vibrancy issues of the previous game. Almost every background of every galaxy had great colors, either due to an actual atmosphere making the sky blue, or a stream of galaxies breaking up the black. It drove the game a bit more away from the space theme, but it felt more like a real Mario game. Add in the levels that homage different parts of Mario History (The SM64 level, the levels from Galaxy redone, the Supermassive galaxy being a bit of an homage to SMB3, Super Mario World with Yoshi, etc.), and you have a solid game. The perfect blend of 2D gameplay and 3D also helped with some of the perspective problems people had with certain parts of Galaxy, and helped focus the game a bit in those sections. Even the bird flying was intuitive, even if getting some of those prankster coins were fucking annoying. I also liked how they put the Green Stars in instead of making the "play again as Luigi" choice again from Galaxy 1. In general, the gameplay and color scheme of the game was spectacular.
However, the plot was a bit weaker, even if I really appreciate them tying Rosalina in the way that they did, and felt like a return to form. Also, with the atmosphere of the areas being a bit more down to earth and not as obviously spacey, it felt like Super Mario 64 (which isn't bad, but just not quite Galaxy). Galaxy 1 was overall more mesmerizing, but I feel I had more fun playing Galaxy 2.
If they were to eventually make a Galaxy 3, I hope they take cues from both games, and make it more well balanced, almost like a Zelda game.
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