This is not only my all time favourite Mario game, but it's also objectively the best Mario game ever made by Nintendo.
TBH this game is absolutely FLAWLESS. Graphics are good, it's hard to believe that this game is in 480p(HQ) instead of 1080p(HD) when you see how beautiful it is. The ost is the best thing I have ever listened to since I played Mario games. It is a complete orchestra that tries to fit the music to the ambiance, the music in the sequel is a lot better than the one in the prequel, that isn't saying much since Mario Galaxy 1 already had an excellent ost.
The level design is like Mario Galaxy 1, but improved. It was innovative as heck, even though this game is not as innovative as MG1 since it's literally the same system as Mario Galaxy 1 it still has an improved level design nonetheless. Mario Galaxy 2 uses different planets on which you can do things before letting you get through with the gravity gimmick that is well thought for many circumstances. There are even more levels that plays with the gravity, the likes of the house level that suddently changes its gravity depending on what you do and where you go.
You have completed the game? No problem, it's time for you to beat some comets and the world s which are what made the game hard and more frustrating. This game unlike the newer Mario games challenges you and I'm always ready for challenges.
This game is not perfect either, it has two flaws (yes I said that it was flawless, now quiet): 1. The story is the same like 95% of the Mario games unfortunately, with an inch of difference, but in the end it's still the old "Go save the princess lul". 2. The camera is crappy. Note that despite these two flaws, the game is just too great to get a 9/10. Maybe we can also add that it uses the fucking quick escape block from New Super Mario Bros Wii, but instead of the block, it's Rosalina the one that helps you (She doesn't help you if you're trying to beat a comet though).
It is also very unfortunately the last good Mario game made by Nintendo. Buy it before it becomes unavailable, buy this masterpiece it is worth it.
EDIT: Some people judged this game to be bad because it is a "copypasta" of Mario Galaxy 1. Mario Galaxy 2 is still great, imagine MG1, but with an improved level design, more challenges and a better ost.