Super Mario Galaxy 2
Is Mario Galaxy 2 the best Mario game ever? Probably! In terms of "pure" video gaming, without stories or dialog or character development, this is absolutely as good as it gets.
It's difficult to nail down particulars - SMG2 is just great. The gameplay mechanics are solid. The world designs are delightful. The variety and pacing are perfect. Looks awesome, sounds awesome. It's more challenging than the first game, but can also offer more built-in help if you need it.
Also, Throwback Galaxy. Yes!
Better than: Super Mario Galaxy
Not as good as: ???
Probably also better than: actual space travel
Progress: 71 stars, beat Bowser