After just beating the Special World, I can confidently say that this is the best 3D Mario game. The Galaxy series was really good, but I didn't have any desire to keep playing to collect all the stars. This game I actually got all the Star Coins and everything.
To resurrect this old topic, just bought this game today due to the 2012's 2011 award - and it is is amazing. I was really skeptical initially as I really don't care about Mario and am bored of the NSMB series, but this is the best Mario game I have played in a long time. I am blown away by it's great quality in all departments.