I'm going to try to keep this brief. I have played and beaten every single Super Mario Bros. game multiple times. As I understand it, Super Mario Bros. 3, Super Mario World, and Super Mario 64 are largely considered to be the best of the Mario games. Having recently beaten Super Mario Galaxy 2, I am quite comfortable in saying that it is as good as any of those were in their respective times. No Mario game, apart from the original Galaxy, comes close to matching the quantity and variety of levels, challenges, and styles of gameplay present in Super Mario Galaxy 2.
The sublime blend of 2D/3D platforming is almost perfect. The music is fantastic, with plenty of 'homage' to previous Super Mario games. The graphics are, by far, the best available on the Wii, with an impressive frame rate complimenting the action. The difficulty is just right, and the hub world/level select changes are welcome and intuitive. The boss fights, especially against Bowser, are epic and thrilling. I was skeptical with the critical reception the game was getting, and it does have a bit of a slow start. But, it didn't take long to pull me in and never let go. If you have any sort of love for Mario platformers, do yourself a favor and buy this game.